This is a zipper that is not infinite in length.
This style of zipper has a beginning, a middle, and an end.
When you close this zipper, it closes.
When you open this zipper it opens, but only so far, and then that's it, you can't take it any farther. At least with this kind of zipper you know what you can get away with.
You often find these zippers in pants, in the fly.
The fly is also called the "access port", "redneck air conditioning", "pickle hole" (maybe because of the warts), or "worm trap".
Originally the fly was just a flap but that wasn't good enough for city wear, so someone added teeth.
Likewise, toothiness is a characteristic of one of the other major uses of the closed-ended zipper, and that is on pullover shirts and such. Catching yourself in this upper-level zipper doesn't grab your attention in the same way the fly zipper does, but is exceptionally thrilling if you have a beard.
Most sleeping bags have a closed-ended zipper too, only there the real thrill is getting it jammed halfway up as you return to bed in the dark, on a freezing night, just after you've gotten up to go pee and have accidentally wet yourself in the wind.
This is called "adventure", which is a related topic, in case you couldn't tell. But it is.
As always, Effort or Eff it. No sniveling.
Source: How to talk in the woods.