It's neither a language nor a tool*. The basic idea is that when you do a design you need to tell other people what's going on. So there should exist something that all the people can understand. UML is such a standardized notation. It's very helpful. If you do the design you need to know how to represent it in UML. If you implement the design you need to understand it. It's already an industrial standard. If you know it(perhaps put in your resume), you look more professional.
*UML is tightly related to Rational(Rose). But itself is just a spec. Rational added some features to the software such as automated code generation. But mostly it's just marketing thing. I don't hear many people use it.