/Master in game will bring up a list of the things you have to do to complete your current Master Level. The information given about the encounters is sometimes vague, as the Arbiters have not yet unlocked all of the secrets of Atlantis. You must explore and adventure through the Trial Planes in order to discover some of the things you must do.
Each Master Level consists of several encounters, and potentially some special PvE or RvR tasks. There are 10 Master Levels total.