What does it mean to "guard the sense doors"? What is the practical application of this concept in daily life? For example, does this concept mean that a heterosexual male should entirely avoid looking at women?
Yuttadhammo Bhikkhu answers a question about "guarding the senses" in this video (at time 34:40).
Guarding the sense doors is a way of protecting ourselves from being overcome by passion for the sensed object (which leads to dukkha). In his video Yuttadhammo mentions three ways to do this.
Physically avoiding the object (in relation to your example, when on alms round monks may keep their eyes facing down to avoid lust from seeing a beautiful woman).
By developing samadhi the mind is disconnected from the sense doors (only the sixth sense "thought" is present)
By developing insight we know the inherent dukkha of sensed objects, therefore no passion arises for them when they are sensed.
The third approach is part of the goal, the first two are training practices to help us maintain a calm mind to achieve the goal.
With each contact your perceive the experience: a pleasant, unpleasant or neutral feeling arises which you also perceive as either favorable, unfavorable and neutral. "Guarding the sense door" is you are aware of the sensation that arose and the perception that arose. Ideally you should be equanimous towards this experience, i.e., not attached or averse to the sensation and stimuli and also realise the evaluation you have given and feed the perception. If not (i.e. you react with craving, aversion or ignorance) then fabrications form: in which case you have be be aware of the fabrications also. These manifest as physical feeling in the body, having the characteristics of the 6 elements; and the mental component has the content 50 cetasikas (52 - 2 = 50 as feeling and perceptions are also cetasikas) -- see section on The Cetasikas in The Abhidhamma in Practice by N.K.G. Mendis
Also note the initial experience, which even if you are have been equanimous has a bodily aspect which you have the characteristics of the elements. The sensation is three-fold (are pleasant, unpleasant or neutral). Likewise the contact, as well as the subsequent reaction of craving if present, has an impact in all aspects of the 4 Foundations of Mindfulness -- for more details on this see Satipatthana related Suttas and literature.
The framework to guard your senses is outlined in the Cha Chakka Sutta though there are more comprehensive dispositions also in other Suttas which covers other aspects also e.g. Satipatthana Sutta Dhammanupassana section.
LATENT TENDENCIES ARISING THROUGH THE EYE. Bhikshus, dependent on eye and forms, eye-consciousness arises.
When the three meet, there is contact. Dependent on contact, there is what is felt as pleasant, or as painful, or as neither pleasant nor painful.
When one is touched by a pleasant feeling, one delights in it, welcomes it, remains attached to it. Thus one’s latent tendency of lust (rāgânusaya) lies latent.
When one is touched by a painful feeling, one sorrows, grieves, laments, beats one’s breast and falls into confusion. Thus one’s latent tendency of aversion (paṭighânusaya) lies latent.
When one is touched by a feeling that is neither pleasant nor painful, one does not understand it as it really is, the arising, the passing away, the gratification, the danger, and the escape with regards to that feeling. Thus one’s latent tendency of ignorance (avijjā’nusaya) lies latent.
Bhikshus, that one could make an end of suffering here and now, without abandoning lust for pleasurable feelings, without removing aversion towards painful feelings, without uprooting ignorance towards feelings that are neither pleasant nor painful this is IMPOSSIBLE.
Abandoning the latent tendencies
ABANDONING LATENT TENDENCIES ARISING THROUGH THE EYE.
Bhikshus, that one could make an end of suffering here and now, having abandoned lust for pleasurable feelings, having removed aversion towards painful feelings, having uprooted ignorance towards feelings that are neither pleasant nor painful this is POSSIBLE.
Also in many other suttas the following passage appears. The implication is do not crave and grasp the sensory experience.
Here, bhikshus, when a monk sees a form with the eye, he grasps neither its sign nor its detail.
So long he dwells unrestrained in that eye-faculty, evil, unwholesome states of covetousness and displeasure might overwhelm him, to that extent, he therefore keeps himself restrained.
He practises the restraint of it. He guards the restraint of the eye-faculty, he commits himself to the restraint of the eye-faculty.
Nimitta and Anuvyañjana by Piya Tan
Sense-control is to experience things with mindfulness & wisdom.
Sense-control, too, has its nutriment; it is not without a nutriment. And what is the nutriment of sense-control? 'Mindfulness and clear comprehension,' should be the answer.
For example, does this concept mean that a heterosexual male should entirely avoid looking at women?
If I can add to the other answers, a book about Buddhism (that I read once long ago) had a photo (of a monk with a fan), whose caption said that Burmese monks use their fans to hide their eyes (e.g. hide from things or from people they don't want to be tempted by the sight of) when they're out in the world.
See this comment, I think that may be one technique among many.
“This is Nanda’s guarding of the doors of his senses: If he should look to the east, he looks focusing his entire awareness, (thinking,) ‘As I am looking thus to the east, greed & distress, evil unskillful qualities, will not flow out.’ That’s how he is alert there. If he should look to the west… the north… the south… above… below… to the intermediate directions, he looks focusing his entire awareness, (thinking,) ‘As I am looking thus to the intermediate directions, greed & distress, evil unskillful qualities, will not flow out.’ That’s how he is alert there. This is Nanda’s guarding of the doors of his senses.
‘Come, mendicant, guard your sense doors. When you see a sight with your eyes, don’t get caught up in the features and details. If the faculty of sight were left unrestrained, bad unskillful qualities of desire and aversion would become overwhelming. For this reason, practice restraint, protect the faculty of sight, and achieve restraint over it. When you hear a sound with your ears … When you smell an odor with your nose … When you taste a flavor with your tongue … When you feel a touch with your body … When you know a thought with your mind, don’t get caught up in the features and details. If the faculty of mind were left unrestrained, bad unskillful qualities of desire and aversion would become overwhelming. For this reason, practice restraint, protect the faculty of mind, and achieve its restraint.’
“And what, Ānanda, is the perception of abandoning? Here, a bhikkhu does not tolerate an arisen sensual thought; he abandons it, dispels it, terminates it, and obliterates it. He does not tolerate an arisen thought of ill will … an arisen thought of harming … bad unwholesome states whenever they arise; he abandons them, dispels them, terminates them, and obliterates them. This is called the perception of abandoning.
Your question is right at the center of "right effort" in the Noble Eightfold Path. Buddha talked about it many times in different ways.
And what is right effort? Here the monk arouses his will, puts forth effort, generates energy, exerts his mind, and strives to prevent the arising of evil and unwholesome mental states that have not yet arisen. He arouses his will... and strives to eliminate evil and unwholesome mental states that have already arisen. He arouses his will... and strives to generate wholesome mental state that have not yet arisen....