Protected batteries have a protection circuit built into them. (IPC chip, integrated protection circuit) if I remember right. Kind of like regulated mods have a circuit that keeps them from short circuits etc.
They aren't good for modern mods though as the amps with them are dangerously low. We used them long ago in vaping though, But it was when the ohms were really high, like 3ohm cartomizers and such and the watts really low.
They are used mostly in flashlights now.