Fall speed is the rate at which characters fall to the ground following a jump or fall.
A character's fall speed can have a great impact on the effectiveness of their fighting style in combat.
For example: Fox has a fast fall speed, allowing a few aerial moves within a quick series of jumps. Other characters, such as Jigglypuff and Luigi, have a slow fall speed allowing them to throw out a series of quick combos and attacks while staying aloft in midair, making them more suitable for aerial combat in most cases.
Just because a character has a fast fall speed does not make them heavy. Fox naturally falls faster then bowser, yet is much lighter. The same could not be said for a character who picks up a metal box: Their weight and fall speed will increase dramatically.