Your air hose connects to one end - the fitting is designed to force the air between the PVC pipe center & the outer membrane; the air then passes through the slots in the membrane and escapes into the water making bubbles.
The end of the diffuser opposite the air connection is open, which allows water to fill the center of the pipe and weight it down. The net result is a large diffuser that injects plenty of air into the water without requiring additional weights or ballasts.