Knights is a multiplayer game in which players must explore dungeons, fight vampire bats, zombies and each other, and try to complete a given quest. The first player to complete the quest is the winner.
Knights is based on the Amiga game of the same name made in 1994. Compared to the original, several new features have been added, including Internet and LAN game modes, support for more than two players, quest time limits, etc.
If you just want to get started quickly then I recommend playing through the Tutorial, which can be accessed from the main menu when the game starts up. This explains the controls and should give you enough info to start playing a few quests. For more details you can then refer back to the manual as needed.