A boom vang or kicking strap (kicker) allows vertical adjustment of the boom.  It is an important tool in shaping the main sail.  

The vang typically runs from the base of the mast to a point about a third of the way out the boom.

Tensioning the vang tightens the leech of the sail, flattens the sail and bends the mast.  By controlling leech tension, the boom vang is one of the three methods of controlling sail twist.  The other two are the main sheet and the traveler.  

Cruising sailors use the vang to keep the boom from rising downwind.

While under sail, the opposite force to the vang is supplied by the sail itself. When the sail is reefed, a topping lift supplies the upward force on the boom. Some line vang systems incorporate a piston to provide the topping lift force and to damp oscillations.