A lip mask is a gel mask that is worn on your lips for a set period of time. Some lip masks are designed to be worn overnight, while others are worn for a shorter period, say fifteen minutes. The purpose of a lip mask is to hydrate your lips, which also makes them look fuller and plumper.
Although it's not a mask in the true sense, the term has come to refer to any product that you leave on your skin for a while. They work in a similar way to a lip balm, but are easier to apply, more intensive, and last longer.