A soil stack, or soil vent pipe, is the main vertical waste pipe on the outside of a house or other building. The soil stack transports waste, sewage, and greywater from the building to another underground soil pipe, which joins the main subterranean (below-ground) sewerage system for the area.
Essentially, the soil stack removes all waste from toilets, sinks, baths, and showers in your building to the main sewer. The vent draws in air to speed up the removal and releases smells.