A cross connection is a connection between a potable drinking water pipe and a non-potable source. For example: you’re planning to spray weed killer on your lawn. You hook up your hose to the faucet on your house and to the sprayer containing the weed killer. If the water pressure drops at the same time you turn on the hose, the pressure change may cause the chemical in the sprayer to be sucked back into your home’s plumbing system through the hose. This is called backflow and could contaminate the water in your home system.
Water utilities deal with this issue on a much larger scale – imagine if your hose were connect to a fire hydrant or a public access faucet (e.g. a campground), then the weed killer would be sucked into the public water supply. Backflow from customer service connections is of concern to water utilities, and has been shown to occur in 1.6% of all meter reads and in 5% of homes with backflow-sensing meters.  Journal AWWA