When you slip, you slide or skid unintentionally. A slip is also a kind of underwear you wear under a skirt. A Freudian slip, on the other hand, is a slip of the tongue.
You can slip on a damp or icy surface, and you can also slip by making a mistake or getting worse, as in "If the company doesn't sell more candy, their profits will slip." It also means "move in a sneaky or quiet way," so you might slip out of a birthday party before cake is served. As a noun, a slip can be a mistake, a fall, a piece of something (especially paper), or a skirt-like type of underwear.