The figure eight stopper knot. One rope, four steps.

1. Cross the end over the standing part to make a "Q".
2. Pinch the top of the loop and twist it once, away from the tail.
3. Pass the tail up through the upper loop from behind.
4. Pull both ends to tighten.

It stops the rope from running out through a block or a fairlead, and it is easy to untie after a load. One of the first knots to learn on a boat.