Slippery (sometimes referred to as slick, or ice physics) is a term used to describe certain platforms or floors throughout the Super Smash Bros. series on which characters can slide and have less traction than normal. In Brawl, slippery platforms significantly increase the chances of prat falling.
The Ice Climbers have the unusual property of being able to walk along the ice with no loss of traction, but otherwise have lower traction on regular floors. Luigi from Super Smash Bros. to Smash 4 and Mii Gunner in Ultimate have the worst traction in their respective games, which bodes well for them in certain aspects of competitive play.
Stages with slippery platforms
Other sources of slipperiness