What are clipless pedals?

Clipless pedals: A pedal that is equipped with a spring mechanism and can be “clicked” into using small, screwed attachments (cleats) on cycling shoes.

Clipless pedal:
 A pedal that is equipped with a spring mechanism and that you can "click" into using small, screwed-on attachments (cleats) on cycling shoes. This creates a firm connection between the pedals and the shoe. This means that you can not only push but also pull when pedaling.

Relevant articles:
Changing bicycle pedals