Clipless pedals are about the most efficient way to attach your foot to the pedal. The rider steps into the mechanism and can then apply pedaling forces in the upward,downward, forward and rearward directions. To release from the pedal, the rider simply needs to twist their foot.
The term clipless is a bit confusing, since many would describe the mechanism in a clipless pedal as a clip. However, they are known as clipless because they are an alternative to the original toeclip pedals that use straps and a toe cage to hold your foot to the pedal.