Well, the name is sort of misleading. It used to be that pedals came in either "platform" (which are just plain old pedals) or the type with clips and straps. Basically a platform pedal with a strap that goes over your foot. That allows you to both push down on the pedal and pull up on it. That works but they are tricky to get in and out of since they have to be somewhat tight to work effectively.
Clipless pedals look sort of like platform pedals, but give the advantages of clips and straps. They have a cleat that attaches to the bottom of the shoe which clips into the pedal. This allows you to both push and pull on the pedals, but they are easier to get in and out of.