Palm spin is a common free running trick, done on benches, small walls, rails, etc. The main point of it is to rotate above the obstacle using your hands. The performer stands opposite the obstacle (or an angle), puts his hands on the obstacle and does a spin on his arms, while he is in a tucked position in mid-air. So the spin is usually about 180°/270°.
This differs from a wall spin as wall spins are performed on a vertical surface, whilst palm spins are performed on the top corner of obstacles.
One-handed variation also exists.