The Clove Hitch Knot is one of the most useful hitches aboard a boat. It's probably most frequently used to attach a fender line to the guardwire or rail, but it can also be used to attach a line to a spar or another rope.
It's not a particularly secure hitch and can work loose if put under intermittent strain, particularly if it's applied from different angles.
The Clove Hitch Knot can be made more secure by taking the working end making a couple of half-hitches around the standing part in much the same way as the Round Turn and Two Half-Hitches.
Some dextrous types can tie the Clove Hitch with one hand!
Make a turn with the working end passing over the standing part;
Make a second turn...
...passing the working end back under its own standing part.
Finally, cinch the knot up tight. It's as simple as that - you've tied a Clove Hitch Knot.