A squeegee is a tool that's used for cleaning windows. You can wash your windows without one, but a rubber-edged squeegee makes cleaning a lot easier (and more fun — because you get to say the word squeegee).
The word squeegee first appeared among sailors, describing a leather-edged device for cleaning a ship's deck. In 1851's Moby Dick, there is a reference to a "leathern squilgee." The word probably comes from the now-obsolete verb squeege, "to press." This tool eventually evolved into the rubber and metal (or plastic) version we know today, used for printing, photography, and window cleaning.