A penny loafer is a specific style of loafer (a loafer is just a slip-on shoe, usually leather). Penny loafers have a strap across the front of the opening where your foot goes in. That strap, which is kind of like a pair of lips, has a slit or gap down the middle running across your foot. It’s designed in such a way that it can hold a penny (or often a dime) or other small coin by inserting it between one side of the strap and the shoe, sliding it in, and then tucking the other side of the coin under the other side. If you go to Google Images, you could find specific visuals.