A pissoir is a special bathroom for boys and men where they can go pee without having to sit down on a toilet. It's usually found in public places like parks or on the street.
It's like a small outdoor room or a little wall with a drain at the bottom where the pee goes. Boys and men face the wall or the drain and pee into it.
A pissoir is different from a regular bathroom because you can't do anything else in it except pee. It's not very private because people can see you from the outside, but it's important to use it when you need to pee and there are no other bathrooms available.
Just think of a pissoir as a special bathroom for boys and men where they can pee quickly and easily without any fuss.