megapenguinx said:
I got one once that said, "Don't use cheese."
It made no sense and was sent to me by someone after a Halo match.
The term means, essentially, something that is technically within the rules but seems like it should be considered cheating. I.e., cheese and cheat sound similar. As an example, a lot of people whine about the sandman from TF2, but it's not cheating because it's official. Those people would consider it cheese.
I don't have Halo 3 so I don't know if there is any cheese in there, but most likely the person who sent that to you is a whiny *****. If something is legal, it's legal. And he should be sensitive to the metagame.