Wikipedia defines userboxes as "small coloured boxes that allow users to add small messages on their user page". They usually include one or two sentences of text and one small, 40 x 40 pixels sized image.
Most common usage of userboxes is to display a users' language skills and other skills. On the right side, there's an example of a userbox. A small box that states that a user knows how to program in PHP programming language. Skills are not the only thing that you can show off with userboxes. You can show off your real-life profession, favorites (such as favorite color or drink), education, political thoughts...only the sky is the limit!