Robin Williams was doing a show and in the middle of it the "Retreat Ceremony" occurred. From what I'm able to search the ceremony marks the end of the official duty day and respects to the flag is shown as the flag is lowered. It was really neat seeing Robin Williams' response: https://www.youtube.com/watch?v=QD9QAAEfQEA
TL;DR Robin Williams shows respect for military ceremony.
TL;DW Robin Williams shows respect for a military ceremony. Crowd explains the ceremony. Williams explains reaction to the event at the end of the video.