You are in command of a turret. Helicopters drop down paratroopers, you must shoot them before they reach the ground. If three of them land safely, they will destroy your turret.
Paratrooper is one of the three games written by Greg Kuperberg when the IBM-PC was still very new, but unlike the other two, J-Bird and PC-Man, which were recreations of well-known coin-op games, this one was based on an Apple ][ game: Sabotage by Mark Allen, 1981. Sabotage may have been inspired by the arcade game Missile Command, but there are some significant differences. In any case similar games are usually called Paratrooper-like and not Sabotage-like games. Greg Kuperberg, by the way, was fifteen years old at the time and would later become a renowned mathematician.
There is a (probably later) 8-bit Atari game Paratrooper, which however has nothing to do with Greg Kuperman's game and is not a Paratrooper-style game at all.