What are the cheat codes and what do they do?
Cheat codes are codes that hack the game to give you an advatage. They can give you infinite currency, ammo, chage NPCs in a variety of ways, reset things, give you items, make you invulnerable, etc. Every game has different cheat codes associated with it though some are univeral throughout a genre, like "noclip" in FPSs.
Some players look down on cheats and those who use them because they believe it ruins the experience but sometimes hacks can be useful, namely if you're stuck, want to test things, skip the grind on a game that is still in development, etc.
That said, hacks on multiplayer/online games is a big no-no, aside from being illegal and risking a perma ban, you're ruining the game for everyone else. In single-players though, most players don't really care about what you do.