It's just what it sounds like: the water is flowing in the opposite direction from its normal flow. With the direction of flow reversed due to a change in pressures, backflow can allow contaminants to enter our drinking water system through cross-connections.
A potentially hazardous cross-connection occurs every time someone uses a garden hose sprayer to apply insecticides or herbicides to their lawn. Another cross-connection occurs when someone uses their garden hose to clear a stoppage in their sewer line.