A plumbing system performs the simple job of supplying water to the house and removing its waste water.
Because a plumbing system ties into a sewer/septic source, it must prevent the possible danger of sewer gases seeping back into the waste pipes. Vents and drain traps handle that problem. The drain-waste-vent system transports all the used water and waste from the house to the septic/sewer system. It's a network of drain pipes that runs to all the sinks, toilets, baths, showers, and washer.
Most newer waste systems use rigid plastic PVC (polyvinyl chloride) or ABS (acrylonitrile butadiene styrene) pipe that are sealed with glue. Older homes generally have had cast iron pipe sealed with lead solder.
Today's homes may utilize cast iron pipe sealed with neoprene in some places as a way to avoid the noise plastic creates when water is draining through it. The soil stack is the main component of the waste drain. It's a vertical "stack" of pipes that starts in the basement /crawlspace floor or wall where it's connected to the outbound sewer/septic line.
The top end of the stack acts as a vent. It extends vertically out through the roof, allowing gases to escape outside and also helps promote drain flow by drawing air inward.
A plugged vent can trap dangerous gases and inhibits drainage; similar to plugging a drinking straw with your thumb to hold liquid. Make sure the vent doesn't terminate in the attic. Trapped sewer gases can be dangerous, stink and cause serious structural problems. And a system without a vent may actually suck water out a sink's trap, or do the reverse and fill the sink with water when another fixture drains. A trap blocks sewer/septic gases. Without one, sewer gases can flow up the stack, drain pipes and come out wherever there's a drain. A trap looks like an "U" and is installed below the drain.
When water drains, the trap's shape causes a small amount of water to remain in the bend. That water blocks any gases from moving up the pipe and entering the room.
NOTE: Traps are needed on all drains. That is, sinks, tubs, showers, washers, floor drains all need to have a trap in their drain lines. In most cases, a toilet has a built-in trap and doesn't require a trap in the drain line.
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