As their name suggests, penny slots are slot machines that can be played for as little as one cent per spin. They're a pretty old model, and, even though lots of modern casinos have kept them, most slots nowadays have more than one payline, sometimes over 50.
That is the exact idea behind penny slots! These slot machines are played at one cent per line, adding up to just a few pennies per spin. Prices of penny slots have remained low, though now cost between 25 cents to even 50 cents per line.