Feuer frei generally means "Open fire!" In this case, the lyrics refers to both a gun, and literally fire. The song talks about impregnating a woman in a way having to do with fire, stating "a hard thrust (that gives off sparks) into her womb." The lyrics also contain a line stating that "the burned child is dangerous" probably meaning the child that was born from fire.
On the other hand, open fire also refers to a gun. "Bang! Bang! Feuer Frei!(Open fire!)" This may also be the reason that an image of a gun is on the cover of the single.
Dylan - Las Vegas, NV