Limbless animals are without legs so they move by their bodies.
Generally their body generate propulsive force (force which move the body forwards) to move on.
The waves is generated inside the body of limbless animal; if it is against the direction of motion then it is known as retrograde waves, and in direction of motion then direct waves.
Some animals like snails and mollusks secrete mucus underside their body to decrease the friction while locomotion, they move by rippling movement of body by producing direct waves. While earthworm move by retrograde waves showing alternate swelling and contraction of body. Snakes moves by producing lateral waves down their body which allow them to move in opposite direction.