I've come across a few references to Magga phala and can only find obscure half explanations of what it is referring to and some of the explanations seem to be contradictory.
Magga is Pali for "path" or "road", and phala is Pali for "fruit" or "result". Thus it is a compound word that can be understood as the result obtained by proceeding on the path to enlightenment. This is the general use of the term, whenever one refers to the benefits of enlightenment.
It can also be used as a precise technical term by some, but before I get to that I will take a moment to explain something about the path, and its destination.
Many Buddhists believe in a graduated path towards enlightenment. The Theravadan belief describes a very precise (some would even say too precise, but really there are always those who cast doubt on anything in any religion) mapping of these states, where, upon attaining each of these states one is expected to have graduated, and having locked in their attainments, cannot fall back to lower states of being.
The Sutta Pitaka classifies the four levels according to the levels' attainments. The four Ariya (noble) states are, Sotapanna (stream-enterer), Sakadagami (once-returner), Anagami (non-returner) and Arhat (completely unbound).
A Stream-enterer is free from:
- Identity view
- Attachment to rites and rituals
- Doubt about the teachings
A Once-returner has greatly attenuated:
- Sensual desire
- Ill will
A Non-returner is free from:
- Sensual desire
- Ill will
An Arahant is free from all of the five lower fetters and the five higher fetters, which are:
- Craving for fine material existence
- Craving for existence on the level of formlessness
The Visuddhimagga is a commentarial book written by Buddhagosa in Sri Lanka almost a millenium after the Buddha, and is the reference text for many when dealing with intricacies in the stages of the path leading to each of these 4 graduations or path attainments.
Like a package tour covers the key tourist attractions in a foreign land, the Visuddhimagga lists the path in panoramic terms as the 7 stages of purification, and further subdivides them into specific terms as including 16 stages.
One hasn't simply not visited Paris, merely because one hasn't hit a key tourist attraction like the Eiffel tower; and one can visit Paris without using a specific tour operator. Similarly, one can proceed on the path without noticing clearly that one has crossed some of these stages, and one can notice other things not covered in these 16 stages, and one can even remain ignorant of the Visuddhimagga and attain these attainments. So I would say this list is a good way to examine the path, but not the only way.
Following attainment of each of the stages of enlightenment, several final stages of mind arise, of which two significant ones are, Magga and Phala.
Path Knowledge (magga-ñana)
It is followed immediately by knowledge that abides in that same Nibbana, which is void of formations since it is the cessation of them. This is called "path knowledge." It is also called "purification by knowledge and vision."
Fruition Knowledge (phala-ñana)
That again is immediately followed by knowledge that belongs to the final stage and continues in the course of its predecessor. It abides in that same Nibbana, which is void of formations since it is the cessation of them. This is called "fruition knowledge."
For an approachable examination of enlightenment in more detail, please see Mind like Fire, Ven. Thanissaro Bhikkhu.
@EyeArrow mentions Ven. Mahasi Sayadaw's "Nature of Nibbana", I've not read it, but it looks interesting, so I'll probably read it soon.
Magga means Path. Phala means Fruit. They are both supramundane (lokuttara) consciousness.
This is what A Manual of Abhidhamma says:
It is called ‘magga’ because it arises, destroying the passions. This Path thought-moment arises only once in the course of one’s lifetime, and is immediately followed by two or three ‘Fruit’ (phala) moments
. . .
Nibbàna . . . is termed supramundane, and is to be realized by the wisdom of the Four Paths. It becomes an object to the Paths and Fruits
There are four types of Path and Fruit consciousness. A pair for each of the levels of enlightenment (stream-winner, once-returner, non-returner and arahant).
Each Path moment can only be experienced once but Fruit moments can be experienced by non-returners and arahants immediately after emerging from cessation-attainment (Nirodha-samapatti).
Edit: Just wanted to add that magga/phala are moments of nirvana. That is to say, the senses (physical and mental) stop coming into contact without outside objects. Said more conventionally, sense perceptions turn off. It is like blinking but with all the senses simultaneously.