It's literally a big middle finger to the American government. It's America's flag but with some black stars hanging out amongst the stripes:

"The three black stars in the upper right represent the three islands that were disfigured in March 1954 during the 15-megaton Bravo test by the United States. The two black stars in the lower right corner represent where the Bikinians live now, Kili Island, 425 miles to the south of Bikini Atoll, and Ejit Island of Majuro Atoll. These two stars are symbolically far away from Bikini's stars on the flag as the islands are in real life (both in distance and quality of life)."

And in large black letters, the text "Everything is in the hands of God."

Let's just say they didn't appreciate their home being irradiated.