A userbox is a small rectangular box that looks like this. More examples below.
Userboxes can come in many different styles and colors.
Userboxes can have one or two sideboxes.

A userbox (commonly abbreviated as UBX) is a small colored box (see examples to the right) designed to appear only on a Wikipedian's user page as a communicative notice about the user, in order to directly or indirectly help Wikipedians collaborate more effectively on articles.

The first userboxes were introduced to support the Babel inter-language effort by indicating the skills of users in other languages.

Be mindful that like anything else you add to Wikipedia, userboxes you add will likely be public forever, even if you later have your user page deleted. (Like other Wikipedia pages, your user page will be regularly archived in the database dumps and may be archived by third parties such as the Internet Archive.)