For decades, lifeguard agencies in the U.S. and around the world have employed flags to notify swimmers of conditions, to warn of hazards, to identify safer areas for swimming, and to notify beach users about regulated areas. The definition of a red flag is "high hazard." It indicates that rough conditions such as strong surf and/or currents are present. All swimmers are discouraged from entering the water when the flag is up, and those entering the water should take great care.

Source: United States Lifesaving Association