Backflow is when water flows in the wrong direction causing contaminated water to mix with portable water. Imagine all the filthy water that is supposed to be flowing into sewers flowing right back into your clean water supply. Needless to say, water flowing in the wrong direction could cause major health issues. Backflow testing is a preventive method to make sure water is flowing properly.
If you are having backflow issues the issue is most likely with system pressure being too low or too high.