Your lips are the external, fleshy part of your mouth. You have a top lip and a bottom lip. Lips are what make kissing possible.
Lips are useful when it comes to speaking, eating, and kissing. If you say that something is "on everyone's lips," you mean that it's the topic everyone seems to be talking about lately. If an angry teacher says, "Don't give me any lip!" he means "Don't be a smart mouth," or "Don't be impertinent." When you manage to keep a "stiff upper lip," you stay brave even when things get difficult.