Bicycle pedals on average bicycles you might buy at a toy or big box store are usually flat with some rough edges and perhaps reflectors on them. They swivel around a metal rod that attaches to the gear-edged plate that turns the chain and thus propels the bike forward.
Bicycle pedals actually come in a wide variety of styles including clipless, toe clip and formed pedals that look like small baskets for your toes to go in. Choosing the right pedals for your ride style takes some consideration.
Flat bicycle pedals are fine for anyone who just rides recreationally on occasion. Although your feet may sometimes slip off, this doesn’t happen frequently and you can recover as you coast for a little while. Since these flat pedals come stock on most bikes, they are the most cost effective option as well.
Bicycle pedals with toe clips or toe baskets or any kind are sometime created to work best with dedicated bike shoes. These shoes have more rigid soles and wider toe boxes and fit inside the clips or structure of the bicycle pedals securely.
The point of affixing the foot to the pedal is to maximize the effort to propulsion ratio. This means that when you push down on the bicycle pedals, the force from your legs does maximum benefit to the forward motion of the bike. When your toe is clipped in or otherwise attached to the pedals, you do not lose any of the inertia and your foot will never slip off.