BBCode is short for "Bulletin Board Code". It is a markup language which is similar to HTML and it is widely used on forums. So it is also called "forums code". Many forums don't accpte HTML code so you can't use HTML to format your text. Most of forums support BBCode. For example, if you want to use bold text, just type "It is [b]bold text[/b]." then the program of the forum changes it into HTML code "It is <b>bold text</b>.".
Below are most useful BBCode markups: