BBCode (Bulletin Board Code) is a system of tags which allows users to add markup to messages (bold, italic, images, links etc) in a safe way. For example, if text in a post was enclosed like so: [i]text[/i], it would be rendered in italics, [b]text[/b] would be rendered in bold, and so on.
BBCode is enabled by default. To configure it, go to the admin panel, click on "Modules" in the sidebar, and click on "Settings" next to the BBCode module. Here you can change a number of useful options, including whether or not to have a button for a certain BBCode in the editor tools bar, for added usability. See Section 18.104.22.168.1, “What is Editor tools?” for more information on this feature. Explanations of what the options do can be found by clicking on the help buttons next to them.