Firing two shots in very quick succession. When doing a double tap the shooter does not line up (sight alignment or sight picture) the second shot. They line up once, and then pull the trigger twice.
For most shooters (with some training) double taps can be practical and accurate at short distances. In sport shooting this can be a very valuable technique for saving time by clearing close targets very quickly.
I wonder what the range was? If it was fired from an M-4 I'm really impressed.