It is known as the "missing man formation" or "missing man flyby."
It is sort of like a last salute to someone who has died.
It is similar to what is performed at military funerals known as "the last roll call" or "final roll call." There is perhaps no more powerful image in the Armed Services than this one event.
During a roll call, members of that unit will stand in a formation. The person in charge will read off names from a list one by one and the people will answer when their name is called. During the roll call, the reader will come to the name of the person who has died. He will call the name once. Pause. No answer. Then call the name again. Pause. No answer. He will then call the name for a third and final time. When still no answer of here is given, the funeral will begin again with a playing of TAPS.
I cannot really convey through words the emotions that come flooding out at this particular time. Doing my best here to describe it, it just seems like another tradition in the military, but to see it in person is such a moving event that it will make even the biggest men cry.
