Combination was removed from RuneScape after an update, but still exists in-game for those who had obtained it.
The combination is a quest item used in the Death Plateauquest. It is obtained by inspecting an iou given to the player by Harold in lieu of paying a gambling debt he owes from losing a dice rolling game.
The combination is a cryptic list of clues that give the relative locations of a set of coloured stone balls that must be positioned on a device which locks and unlocks the equipment room in Burthorpe Castle. If lost, the combination can be obtained by talking to Harold again.