When water runs down the drain and out into the municipal sewer system, you expect it to stay there, but there are certain conditions that can cause sewer water to travel the wrong way through the pipes. This happens when there is a drastic change in pressure due to either a major leak in a water main or when a fire truck connects to a fire hydrant. When this happens, your home could be at risk for getting contaminated water flowing back in.
There is however something that can prevent this from happening. It is called an atmospheric vacuum breaker. The way an atmospheric vacuum breaker works is by allowing air into the pipes to stop the sudden vacuum from sucking water back into your home.
While this is a fairly simple mechanism, it is vital for the proper functioning of your plumbing system. If there are contaminants in the air where the pipe is located, this could lead to contamination of your freshwater, so you should be aware of the location of this pipe and make sure it is kept in good repair.