Cleat
A Cleat is a small fitting with two projecting horns, bolted to the deck or a structure, used to make fast lighter lines — heaving lines, small mooring lines, flag halyards and the like — by taking figure-of-eight turns around the horns.
Cleats are for lighter loads than bitts and bollards; the largest mooring and towing lines are turned up on bitts. Cleats appear all over a ship and on small craft, where they are the standard way to secure a line quickly and release it under load.
Cleats give the crew a fast, secure way to make fast and let go lighter lines without tying a knot, which matters when a line may need to be released quickly under load. They are simple, but the right size and correct turns are what keep the line held.
What lines are secured to a cleat?
Lighter lines — heaving lines, small-craft mooring lines, halyards and similar — not the heavy mooring and towing lines, which are turned up on bitts or bollards.