I think the Falcon himself described it perfectly:
The mods people have for SSBB are really something, Brawl- and
Brawl+ are very notable mods. Brawl+ mixes the mechanics of Brawl
and Melee making the gave extremely competitive, while Brawl- makes
the game very over-the-top while still maintaining balance.
Brawl- - SmashWiki, the Super Smash Bros. wiki
Brawl+ - SmashWiki, the Super Smash Bros. wiki
But those are only two of countless mods.
Some are crazier than others.