Installing a faucet is a major pain.
It’s simple: There’s a hole in the sink deck maybe an inch around. You drop the stem of the faucet through that hole and put this great big nut on it from underneath.
Except this is all happening in the space between the wall and the sink basin. There’s basically zero clearance for a wrench between there, and you’re on your back in the cabinet under the sink working above your face.
It’s very comfortable.
Getting the nut on there is relatively simple, and spinning it up the stem is usually no problem. It’s just that last half-to-three-quarters turn you need to do to get it tight.
The basin wrench has a long shaft with a T-handle at one end and a spring-loaded jaw at the other. While your friend holds the faucet itself in place, you reach the shaft up, spring the jaws around the nut, and cinch the nut tight.
If you need to remove the faucet, the jaw flips over so the spring is pushing the other way, and you do the opposite.
Don’t buy a basin wrench until you need to install a faucet: There is nothing else you will ever need one for. And even then, wait until you’ve unpacked the faucet and looked at the parts: I’ve recently seen a few faucets that either have a plastic nut that you can tighten sufficiently by hand, or a regular bolt and a weird little forked fitting that holds around the hole without blocking it, that you use a regular nut on this bolt to hold in place. And for that, you need to use a regular crescent wrench or a socket. But there’s plenty of space to do that, since there is no huge faucet stem in the way….