UBB Code is a way of providing limited access to HTML capabilities.
There's a brief explanation of UBB Code in the FAQ. A
link to that section of the FAQ is displayed when you're making a post.
If arbitrary HTML is allowed in posts, users can potentially compromise the security of your board, as well as doing annoying things. For this reason, it is generally a good idea to only allow UBB Code, and not HTML, on your forums.