It is known as bitt knot but the proper name is cleat knot since our boats usually don’t have any bitts.
It might not even be a real knot, since, however you make it, you must always cleat a rope and not fasten in a knot.
Anyway, the operation consists in fixing a mooring rope from the dock to a cleat
Then, it is necessary to delay and make a zero around the cleat basis in order to stop the eventual traction of the boat. Then, draw a eight by crossing the current rope.
To finish the knot, make an overturned half-hitch.
Some sailors use another method which avoids the final overturned half-hitch. It consists in making a OXO, that is a zero on the basis of the cleat, an eight and finally another zero. This way, the knot is easier to untie.
Personally, I prefer the first version; the final half-hitch, in fact, can’t set taut because there is no tension in that point. It might be more difficult if ropes get wet but I have never been obliged to cut ropes to leave a mooring.