A wrench is a tool for gripping things that turn. Like a nut, or a bolt head. (Or a pipe, but there are special wrenches for that that I’ll go over another day.)
The best way to do this is to use a wrench exactly the same size as the nut (we’ll call it a nut. It’s just the same for bolt heads, valves, etc. Anything with a small number of flat sides that is designed to turn). You put the wrench on the nut, and the power of the lever works for you to help you turn it, or hold it steady while turning something else.
But not everyone has a full set of wrenches in every conceivable size. It’d be easier if there was a wrench that you could adjust to fit the nut in front of you. There is such a wrench, of course: The Crescent Wrench. (Technically only wrenches made by Crescent, A Division of Apex Tool Group can be called a “Crescent Wrench”; all others are “adjustable wrenches”. But if you go to the hardware store to buy one, they’re not going to blink when you ask for a Crescent Wrench and pick out one made by someone else. In the real world they’re all crescent wrenches.)
(A word about pliers, which I can hear some of you wondering whether you should just use pliers for turning nuts: Don’t. Sometimes you have to but really: don’t. The pliers will almost certainly damage the nut, which will a: make it harder to turn with pliers, and b: may make it impossible to turn with a wrench. And then you’re just stuck. Please: don’t use pliers to turn that nut. If you must use pliers, use them to hold the bolt while you use a wrench to work the nut. But even that will be damaging the bolt. Use a second wrench. Pliers are for other jobs, not this one)
This is probably in the top three tools you should own, along with a screwdriver and a hammer. Get a 6-inch one to start with: its jaws will open wide enough to handle most anything you’re going to be likely to want to grip at home and its handle is long enough to provide good leverage while being short enough to mostly not get in its own way. You will undoubtedly need a second one soon enough (to hold the bolt while you turn the nut, for instance) and by then you might have an idea or opinion whether your second one should be a 4-inch to get in tighter spaces or an 8-inch for more leverage.
How to use it: adjust the wrench to roughly the right size but too big. Put it around the nut. Adjust it to fit quite snugly against the faces of the nut, then back off a hair. It should be easy to get on and off the nut but impossible to damage the corners of the nut by slipping. Most of them have a little arrow molded into the back end of the handle. The wrench is designed to be turned that direction: if you’re holding the wrench in front of you with the back end of the handle in one hand and the head in the other with with the moving jaw pointed down, the back end of the handle should be moving down as you turn. Naturally, that’s not always possible; sometimes the space is tight and you have to turn a tiny bit, turn the wrench over to get another angle, turn a tiny bit, etc. And that will work. But it’ll work better if you can use it the way it wants to work. If you apply a lot of force to the wrench “backward”, the moving jaw can bind and not be movable. If that happens, tapping the flat of the head against something hard like the floor or a pipe can often free it. Buying a higher quality wrench can minimize this problem; remember what I said about moving parts? I was thinking of the crescent wrench when I said it. The jaw should move smoothly on its track. A little play is good. It makes it easier to adjust and easier to get on and off the nut as you turn, re-place the wrench, turn, re-place the wrench. But it should not be too loose and “wobbly”.
So there you have it: Go buy a good crescent wrench. Or wait until you need one, I suppose. But you’ll be happier buying one at leisure; “in a panic because there’s water everywhere and AIIEE!” is not the best mood for evaluating a good tool.