A buff is a term used to describe the improvement of a character (or aspect of a character), either through updating a game or by releasing a sequel. The opposite term is "nerf" which means to weaken the character between sequel games.
Characters that were poor in comparison to other characters are often buffed in sequels, for balance reasons and to make the character more popular to play. It is also common for individual attacks that were ineffective to be buffed to increase their viability in future games. A good example of a character being buffed in the Smash series is Kirby in Super Smash Bros. Brawl, where he has quicker, stronger attacks, along with increased reach, in strong contrast to his attributes in Super Smash Bros. Melee where he was nerfed.