The King moves one hexagon through the sides or one hexagon through the corner (diagonal). The King must be protected from capture by your opponent. If your King is captured you lose the game.
Note the relaxed aim of xChess is to cause the opponent to resign or capture the opponents King.
This is similar to traditional "Check Mate" which stops short of the capture move, but in relaxed xChess the opponents King may actually be captured to win, and may be captured from a "non-check-mate" scenario. See "Objective" above.