Flippers, or fins as they are sometimes called, are a rubber covering for the foot which helps you to swim faster.
They are based on the fins of fish and have a flat, flexible fin which widens as it extends forward from the toes, allowing more leverage every time you kick.
Coaches often encourage their swimmers to train using fins to help increase kick strength, ankle flexibility and improve body position.
There are different types of flippers.
Short or small bladed fins allow a foot speed closer to your regular kicking speed, medium blades allow a more powerful leg movement, and long bladed fins are used by scuba divers.