I see it everywhere in my book on jhana. but don't truly understand it's meaning or even find a definition.
Fruition is more related to the stages of Attainment. These levels of attainment are described as including four or eight kinds of individuals. There are four [groups of noble disciples] when path and fruit are taken as pairs, and eight groups of individuals, when each path and fruit are taken separately:
• (1) the path to stream-entry; (2) the fruition of stream-entry;
• (3) the path to once-returning; (4) the fruition of once-returning;
• (5) the path to non-returning; (6) the fruition of non-returning;
• (7) the path to arahantship; (8) the fruition of arahantship.
So, Fruition simply means “Coming to Fruition”. It is the moment of the fruition of their path (magga-phala) – be it the stream-entry path, the once-returning path, the non-returning path, or the path to arahantship.
The moment of fruition is the understood experience and results in a turned-around vision of existence. The new understanding at the culmination of the stream-entry path recognises and go beyond the first three fetters (saŋyojana) that bind a being to rebirth, namely self-view (sakkāya-ditthi), clinging to rites and rituals (sīlabbata-parāmāsa), and skeptical indecision (Vicikicca).
The new understanding at the coming to fruition of the Sakadāgāmi or "once-returner," stage is first three fetters (saŋyojana) are fully cut off and that s/he has significantly weakened the fourth and fifth saŋyojana.
The new understanding at the coming to fruition of the anāgāmi or "non-returning" stage is that s/he is free from Kāma-rāga (Sensuous craving) and Vyāpāda (ill will).
A key principle in Buddha Dhamma is cause and effect; nothing happens without a cause or a reason. The word “FRUITION” can be used here in this way: Even if root causes are there, we can stop them from bringing their results by blocking the conditions for them to come to FRUITION (a seed has the potential to bring about a tree, but for that to happen the seed must be given a fertile soil, water, and sunlight). In the same way, kamma vipaka (result of a past kamma) can come to FRUITION only when right conditions for the corresponding kamma seed to germinate are realized. This is why we can prevent many bad kamma vipaka from come to FRUITION by acting with yoniso manasikara or by “being mindful” in seeing life through the ‘Dhamma Eye”.
Fruition is more related to the 4 stages of sainthood. This is realising of the fruit (Pala) of practice. What his means is you realized the path.
As the venerable appearing as @Saptha Visuddhi has given a detailed answer, I will give a simple one. Fruition is a significant milestone in the practice of Dhamma. The final fruit is the Arhathhood which is the elimination of suffering. A dhamma practitioner practices all the components of the Eightfold noble path, which includes development of wisdom until he/she reaches each of the 4 fruitions and finally the Arhanthood (or Buddhahood).
It is said that at each fruition, a taste of nibbana can be experienced. And once that it's experienced, a normal person would no longer have doubt about the Buddha, Dhamma or Sangha. And that person is bound for nibbana. The practice of vipassana removes mental defilements step by step, increasing wisdom, until a person achieves each of these states.
Once one achieved stream-entry, one will know what is what. The same applied to once returner, non-returner etc. In layman term to make clear, if one go to a retreat home/dwelling, the entrance to the dwelling is the path(magga) and the dwelling where one(stream-enterer) stay is fruition(phala). But magga is very very short just like passing by the entrance while phala is the stage where one (stream-enterer or once returner or non-returner or arahant) can be in this stage for long enough depend on the concentration achieved just like dwelling in a retreat home. If pre-determined before meditation session, can stay in the specific fruition stage for a definite period of time.