I have seen some games being called "AAA". What does it mean?
It's not a definition set in stone - but it usually refers to a game published by a major publisher, enjoying a large budget (can be millions of dollars) and a large team working on it over a few years. AAA titles typically also have a lot of publicity.
The term does not say anything about the game quality or sales figure.
"AAA" is an adjective to describe quality. AAA can be be used to denote a major project for a developer, such as a Halo or Metal Gear. AAA can also be used to denote high quality, such as Batman: AA or Little Big Planet. AAA is a marketing term, so it is perceived quality rather than actual quality (although it can be both)
For kicks, an analogy:
AAA is to Games, as Blockbuster is to movies
A fair warning though, not all movies that are considered blockbusters are great (cough Transformers cough)
AAA in many industries, gaming included, means the highest level or best in class. Games produced by AAA studios are generally expected to be of the highest quality.
In sports AAA is typically the highest level of amateur play such as AAA baseball, volleyball, etc...
Developers like Blizzard, Bungee, Rock Star Games, etc... would be considered AAA development houses and titles they release would be considered AAA games.