You're not "stupid" for asking a question, Hackerfloof~ *pats*
A T/M rating stands for Teen/Mature, and it's usually a rating for games, much like G/PG/PG-13 for movies (or, at least that's how it is in the USA).
A game with a T/M rating usually has more suggestive or "mature" themes, such as violence or substance abuse, which parents would probably like to know about before their kids go to play it. It all depends on the game itself, but generally you'll find violence to be the main factor in a T/M game.
