Answer:
It means diver down.
Step-by-step explanation:
This flag is one of many International Marine Signal Flags. It indicates that a diver is in the water and it is used to ensure the diver's safety.
While he/ she is submerged, this flag helps other boats understand that the diver is under the vessel or buoy and they know to keep their distance as to avoid hurting or hitting the diver once he/she comes to the top again.