It's C's equivalent to exceptions, except without any of the stuff that might actually make exceptions safe and reasonable to use in other languages.

There's no automatic cleanup of anything, no built-in way to even register things for manual cleanup, and it doesn't even consistently save/restore the values of your local variables. A local variable that was mutated between the setjmp and longjmp may end up with the old value or the new value depending on where the compiler decided to store it. The documentation does mention this and says that you need to use the volatile keyword to avoid it, but in typical C fashion there's nothing to enforce this, it's just up to the programmer to remember it.

It is extremely uncommon in C codebases because it is extremely dangerous. It makes hazards like pointer arithmetic and manually-terminated strings look like a walk in the park by comparison.