Don't ever be sorry to ask a question. As I understand them, trainers are programs written by people other than the developers that you download and run concurrently with your game and change it. They do various things, some of which are the same as what you would be putting into the game using codes. As for using them, they usually have instructions on the page where you download them, but I don't use them so someone else would have to give you better particulars.
"How could drops of water know themselves to be a river? Yet the river flows on."
- Antoine de Saint-Exupéry