This refers to the on-game forum system or Bulletin Board system. XMR uses Myrddin's BBS code, which has become a bit of a standard across many MU*s.
This works much like a web forum except that you need to type in commands (like everything else on the MUSH) to read and post messages. Some boards are open to everyone and some are locked off to specific groups (e.g. Xavier's School, X-Men, Brotherhood, etc.).