Sewer pipes are on the roof s of homes, and are usually PVC (white plastic) or cast iron. They allow air to enter or leave the sewer system in the house as needed for the drains and toilets to work properly.
When it’s very cold out, water vapor can freeze on the end, plugging it. This will cause a glugging (that’s a word) sound when the bathtub is drained or when a toilet is flushed. It could cause drain traps to empty. An empty drain trap would allow sewer gases to enter the home. These gases, mostly methane and carbon monoxide, cause a bad smell and can be dangerous. They may cause people to become sick with symptoms such as headache, nausea and/or dizziness.
The plugs form more in older houses. The old cast iron pipes carried more heat from the home. And newer homes have attics that are better insulated and thus colder than in older houses.
When I’m inspecting a home, I always look at the plumbing vents for age-related damage and for obstructions, be they tree debris or frost. I’m glad I have a drone camera; looking down a plumbing vent in the winter is hard!
Is this a big deal? Nah. It could be, but there are things you can do.
On cold days, listen for glugging. You can also see frost plugs with a pair of binoculars from the ground. Check all the drains for sewer smell and add water to empty ones. (FYI, you can put AV antifreeze or salad oil in seldom-used drains to keep them from drying out.)
That’s it. Some folks insulate and/or heat the pipes. Menards sells both insulated covers and heat tape.