Windsurfing booms are the primary location for a windsurfer to interface with the sail. The primary setup is known as a wishbone boom. The name is a fairly descriptive one, because a wishbone boom comes off the mast on both sides of the sail. This is needed, because the mast is attached to the board via a universal joint.
A universal joint affords nearly unlimited movement, which means that a surfer in a single session may find themselves on both the front and backside of a sail. No matter where the surfer is in relation to the sail, they need something to hold on to, so that they can adjust sail angle, height, and the like. Windsurfing booms are this location, but they do much more than provide a gripping point.
Windsurfing booms are also one of the main locations where the tension of the sail can be adjusted. The rigging runs down the mast and out to the end of the boom, much like the rigging on a larger vessel. Unlike larger vessels, windsurfing rigging is rarely adjusted mid voyage, though it can be done. It is far more common to set the sail before you set out and then come back to land for adjustments.