Details: This is a poster of a black and white photograph of two girls on a beach holding surf boards. Both girls are wearing bikini bathing suits and their buttocks are peeking out. At the bottom it says 'Beach Bums'.
A bum is a slang term for a homeless person or beggar, and also for the human buttocks. It is used in this poster ironically, as the two girls' buttocks are visible and they are on a beach, and 'beach bums' are people that frequent the ocean.