What is Backflow?
Backflow is the undesirable reversal of the flow of water from its intended direction in any pipeline or plumbing system. Backflow is dangerous because it can allow drinking water in plumbing systems to become contaminated and unusable.
What is a Backflow Preventer?
Backflow preventers are mechanical plumbing devices installed in a plumbing system to prevent water from flowing backward in the system. A properly installed, tested and maintained backflow preventer at the service entrance to a building or property can reliably prevent the backflow of water of an unknown quality from flowing back into the community water system.
Check out this video from the American Water Works Association explaining what backflow is and why backflow prevention is necessary in more detail.