This mark is described as a sign of allegiance to the "beast," a figure of opposition to God, and is enforced by the "false prophet" so that only those with it can buy or sell. It is often linked to the number 666 and represents a choice to reject God and worship man's systems instead. 
Revelation 13:16-1716 And he causeth all, both small and great, rich and poor, free and bond, to receive a mark in their right hand, or in their foreheads:17 And that no man might buy or sell, save he that had the mark, or the name of the beast, or the number of his name.