A single pole light switch only has two screws and they are on one side of the switch. The switch only controls power flow one direction.
A double pole light switch has two sets of two screws one pair on each side of the switch. These switches are used to set up two and three way switching for lights, like when you have a switch at the top and bottom of a set of stairs. These switches control power flow in both directions. With the switch up one side of the switch allows current flow. With the switch down the other side of the switch allows current flow.