Air friction is a measure of how long it takes for a character to stop moving from a sideways force when in midair. It is the aerial equivalent to traction, which only applies when grounded.
Air friction only applies when the player is not holding the Control Stick in a horizontal direction (or the input is being ignored, such as when in hitstun). If the player is attempting to slow down, air acceleration applies instead, and air friction is ignored. This allows characters to be designed with several properties:
It is circumstantial as to whether high or low air friction is good or bad. Low air friction means a character will not slow down as much if the player attempts to use a neutral aerial when moving, and will cause them to travel a bit further during combos. But on the other hand, high air friction helps resist being KO'd to a degree, and it may be beneficial in some cases to slow to a stop quicker without providing directional input.
Air friction values
Super Smash Bros.
Super Smash Bros. Melee
Super Smash Bros. Brawl
Super Smash Bros. 4
* Ryu is a special case. In most circumstances, his air friction is a standard 0.015. However, when he jumps, his air friction is 0, meaning he will not slow down at all unless the player attempts to decelerate.