It is a wing you attach to the bottom of a surfboard so you can hydrofoil (ride the board with only the foil in the water). You need energy to move forward. Th

It is a wing you attach to the bottom of a surfboard so you can hydrofoil (ride the board with only the foil in the water). You need energy to move forward. This can come from a tow boat, a kite, waves, jumping with your legs ("pumping" in TFA) or a small propeller ("e-foil"). It's fast because the drag is really low. Same as any hydrofoiling boat like some ferries or America's Cup sailboats.