In the mundane world:
- Doubt on One's Ability
- You might not undertake beneficial action because of lack of confidence
- Doubt in a Teacher or Doctor
- You will not learn what is been taught or take the treatment given
- Doubt Something Will Work
- If you have doubts on if something will work or has a chance to succeed you will not attempt it
- If you doubt something works for you specifically
- You have confidence something will work but doubtful if it will benefit you. Say you are doubtful a certain fitness routine works on you due to some reason you will avoid such routine or not follow through with earnest
- You have doubt something will work in current times
- If you think something worked in the past but not any more you might not follow through with it in earnest
- You are too old or too young to do something
Similarly when practicing meditation. You have to have confidence in:
- The technique
- The benefits to be reaped
- One's ability to execute and come out successful
- It will work for you
- It will work currently and also regardless of one's age
Also some forms of doubt is wholesome and will not be a hindrance if done in the spirit of free inquiry. E.g. Kesa,puttiya Sutta (or Kālāma Sutta). Also the Buddha mention that doubt is justified in couple of Suttas including Kutūhala,sālā Sutta. As I see it any of these instances are skeptical on what is been heard or taught by others or skepticism arisen due to logical fallacies in reasoning with the accepted norms of the day. Also the people were in a stage prior to doing serious pratice of the path and training. So doubts before serious pratice and arisen from what has been taught or inferred the general acceptable knowledge is justifiable but so when you pratice or regarding the path and training.
In essence being skeptical in any endeavour diminishes your ability to accomplish the end goal. When you initially approach a task or endeavor you can have your skepticism but when your you start executing you ideally should be void of doubt.
More broadly speaking with respect to all the hindrances including doubt. What needs to be done is to starve the hindrances and feed the awakening factors. [(Nīvaraṇa Bojjhaṅga) Ahara Sutta] Since hindrances weekend wisdom and the awakening factors enhances wisdom. [Āvaraṇa Nīvaraṇa Sutta]. The hindrances are also dependently arise based on the unwholesome roots. [(Akusala,mūla) Añña,titthiya Sutta]. Couple of way to overcome them are given in [Nīvarana,pahana Vagga], [(Satipaṭṭhāna) Nīvaraṇa Sutta], etc.
Coming back to doubt specifically. The reason for doubt is unwise attention. [Nīvarana,pahana Vagga] With unwise attention you begin to believe something not inline with reality (more on these perversion see: Vipallasa Sutta) and start doubting the teacher, teaching, benefits of the pratice. Not being able to clearly see the truth of the dhamma, teaching and practice leads to perplexity. Unwise attention should be overcome by seeing or experiencing the truth. [Sabb’āsava Sutta]