a bit more context is needed. It could mean somebody was slipping or about to fall off​ a cliff. Or that somebody was not being as good at something as they usually are.

"you are slipping in class" - as John said, you are not performing as espected. "you slip it under the door" - the act of putting an envolope under a door when there's no post box or "the cat slipped through the cat flap" "if you slip you will miss out on an oportunity" - not paying attention