A sewer stack vent is the vent that you see outside on the roof above the room needing venting, usually a bathroom. As a previous poster has said, if they're designed to code, they're meant to keep sewage out of your home as well as its odor. And the vent that comes out of the ground is the soil vent. What I'm referring to here is in a typical home that's hooked up to a sewer.
Now, for a home that's operating with it's own private septic system, this may be what you're seeing. Nonetheless, it/they operate the same way if they're installed to code. If this doesn't help, you can search the WEB using "sewer vent" as a keyword and get quite a few 'hits' that talk about sewer vents.
My best to ya and hope this helps. And if you're REALLY wondering what these 'stacks/vents' are, ask the builder or a current homeowner in the development in question. In any event, if you find out that they're NOT what we're saying they may be, PLEASE tell us what you find out they are. We can ALL benefit from that information.