The context of the song is that the blond guy, a pre-med student with a nice girlfriend, gets hit in the head by accident and goes anti-social and eventually joins this band. They troll the audience by performing a song with homosexual overtones, to the fury/delight of the crowd. One of the very few Yugoslavian films to deal with homosexuality at all, even in a joking context.
The song starts out with the refrain "I have the best girlfriend" but quickly turns sour with "I don't want the best girlfriend, I want you" and continues with the two singing to each other "I am yours" and "You are mine". Epic trolling.