The video opens with a scene of a cloudy sky. The film cuts to a hill, which then cuts to a scene of a shoreside. We then see a blue ten-gallon drum lying on a bed of pebbles. The film then cuts to a scene of a person playing in the waters of the sea. Stones in the shape of the letter H, a vertical line, and the letter I flash on the screen as the video closes to the ten-gallon drum being blown by the wind.
Flotsam means the wreckage of a ship or its cargo found floating on or washed up by the sea. Jetsam means unwanted material or goods that have been thrown overboard from a ship and washed ashore, especially material that has been discarded to lighten the vessel. Lagan means goods or wreckage lying on the bed of the sea. Derelict means in a very poor condition as a result of disuse and neglect.
With this information, we can assume that the Flotsam/Jetsam/Lagan is referring to the blue ten-gallon drum lying on the shoreside, and the Derelict may refer to the condition of the drum.