Full Moon is a film in Grand Theft Auto: Vice City Stories, a billboard for it can be seen in Viceport. The name comes from the act of mooning, where someone's bare buttocks is fully exposed. The moon on the billboard has a crevice going through the centre, resembling a pair of buttocks.
The film is rated BS, presumably for bullshit. Regardless, the film cannot be seen by the player.
The billboard is placed next to a Unique Stunt Jump, which is used to access the impound in the mission The Audition. If the leap is executed perfectly, the camera will switch to the side view and it should be reminiscent of the famous flying scene in E.T. the Extra-Terrestrial.