This forum uses s9e\TextFormatter, a text formatting library that supports BBCode, Markdown, HTML and other markup via plugins.
If you are not confident with any of those markup systems, consider looking up the documentation linked in this thread before posting on the board. Not properly formatted posts may require OP to re-edit them before being approved.
Markdown syntax is arguably the easiest to grasp, and chances are high you came across it elsewhere already. TextFormatter implements a Markdown parser in the form of Fatdown dialect.
For further reading, see the official Flarum formatting guide, which focuses on Markdown syntax.
BBCode is also supported, so if you come from PHP fora –like vBullettin, phpBB or Xenforo– you shouldn't have problems feeling at home. See the official list of BBCodes supported by TextFormatter.