The chemical bath you run
photographic paper/
film through right after development. A stop bath is used to make certain that the developer is
done doing its
job.
You see, development is a very time specific process. While an extra second or one less second in the developer probably isn't going to kill you, an extra five or ten really could. Using a stop bath makes certain that the development process ends the instant the film/paper hits the bath.
The only reliable stop bath for paper development is a dilute solution of acetic acid. When developing film (where time is slightly less critical), cold water usually suffices.
Because it is an acid solution that you mix up yourself, stop bath is by far the most dangerous thing you're going to be playing with in your darkroom. Getting it on your hands, face or clothes is a bad idea. A worse idea is getting it in your eyes or inhaling it directly. I've never poured stop bath into my eyes before, but that's not necessary for it to be unpleasant. It certainly burns your nostrils and makes your eyes heavily water if you absent-mindedly and violently mix it up in an enclosed room with little ventilation.
A little care for your health goes a long way.