It's kind of a subset of HTML that you can use to format your posts on this forum. If you position your mouse pointer over the buttons at the top of the post creation screen, you'll see descriptions of the BBCode they generate and how to use it.
Although that isn't always very clear, so you might find some kind of BBCode guide useful. Wikipedia has a page on it, for instance: https://en.wikipedia.org/wiki/BBCode
(This forum may not support all the BBCode tags Wikipedia describes--I'm not sure about the "style" and "table" ones, for instance.)