This is a standard Boy Scout knot, and supposedly known to everyone who has ever set up a tent. Despite that, I keep coming across people who don't know it and are thoroughly surprised by its trick property. Basically, it's a one-way sliding loop. You can easily slide it to increase the size of the loop (and therefore shorten the rest of the line), but it locks and won't slide the other way so it won't slacken and loosen the line. Uses include:

1. Spreadeagle ties, where it can be used to change tension on the limbs. But it does require a fair bit of distance between wrist/ankle and bedpost. Or stake in the desert. Or whatever.

2. Suspensions, where it gives a quick and easy method of adjusting secondary positioning lines to arms and legs. But I would never use it for primary load bearing lines!

3. Putting tension on wet ropes put out to dry. My usual method (on hot sunny but breezy days, for preference) is to use some old rope to make a tautline hitch loop around a tree, and to tie a loop a foot or so long in the other end. Then I make a larks head with that loop around one end of the wet rope, just above the thistle knot at the end. I repeat the process at the other end of the rope. Then I adjust the tautline hitches to put as much tension as I can on the rope, and tighten them up a bit further from time to time as the rope dries.
Of course this method requires some trees at convenient distances, but I'm lucky enough to have them. Incurious neighbours also help - neither of mine have yet asked me what I'm doing with all that rope, although I'm sure they must have wondered.

The tautline works well in most kinds of rope, but you may occasionally find some synthetic rope which won't play right. Live with it... It also works with doubled rope, which is handy when doing Japanesey things. It's also tolerant of things like an extra turn or two on the "inside" if you think that might help its grip.

No need for tutorials here, because there are lots of them online. Google will find them. And there's a good Wikipedia piece on tautline hitch variations.