I'm going to give as un-loaded of an answer as possible because depending on who uses "AAA" they could mean different things.
A game that is "AAA" is one with a higher budget which has gone through an extensive playtesting/quality insurance process. "AAA" games tend to have fewer bugs and friendlier user interfaces than games with lower budgets.
On the (potentially) negative side, because "AAA" games require higher funds to make there is an increased chance of executive meddling in the end product, which can sometimes cause the games to lose things considered a barrier to their acceptance at large. This can be things like chopping portions of the game that are considered "too difficult," "frustrating," or "confusing." Because of this, "AAA" may have a negative connotation with some gamers.
Positive: "Wow they really made a AAA game with the Legend of Zelda: Wind Waker. I love the art design and music especially!"
Negative: "Did you see all the features they removed in Oblivion just to make it a 'AAA' game that appeals to the masses? You can't even levitate anymore... that was one of my favorite things about Morrowind!" --- The man keeps us down because he likes to see us frown