BB Code (or vB Code) is a method by which your members can format their messages. Its syntax is similar to HTML, but it has the benefit that you (the administrator) can define exactly what codes are allowable in order to prevent unwanted formatting or malicious use. It is also less prone to break the layout of your forums than raw HTML.
The Custom BBCode manager allows you to define new bbcode tags in order to extend the range of formatting available to your members.
Here is a list of all the default BBcode, https://www.vbulletin.com/forum/misc.php?do=bbcode