I've heard of Halo trainer. What is it and what does it do?

a tranier is basically a cheat. people edit the coding of a game to make it so u have unlim ammo, or infinite health, just make the game easier. if you apply a trainer to a game, it'll basically put cheats on teh game

under the evolution x section there is a guide to trainers that is really helpful, check it out.

A trainer is a modification for a game that allows 'cheats' to run in the game.

Usual modifications are unlimited health, ammo, etc.

They can be patches for executables, or Evox Trainers