In a fighting game, reprensents the strongest characters in the game. Top tiers are usually marked by higher priority moves, hard hitting attacks, strategy variety, speed, and (sometimes) ease. When a game first comes out, the top tiers are usually the easy-to-use cheap characters, and players tend to rely on cheap tactics such as "spamming" a high priority move. After some time, the game tactics begin to develop as people learn new strategies with other characters. Eventually the "cheap" top tiers are replaced, knocked down a level, or accompanied, by new characters. A good fighting game should be as tierless as possible, although this is hard to achieve.
Marvel VS Capcom 2 matches are dominated by top tiers due to the fact that they are so much better than the rest of the characters. In Capcom VS SNK 2, the top tiers aren't so much better than the lower tiers.