The Old Guard is the name familiarly given to the 3rd U.S. Infantry, an active-duty unit based in Washington, D.C. They serve as the President's escort, and more publicly, serve as sentinels at the Tomb of the Unknown Soldier in Arlington National Cemetery.
It's a high honor to be chosen to serve in the Old Guard, as the selection process is rigorous. They take their job incredibly seriously, as you see in the video above: a few seconds into the video, some people disrespect the sanctity of the memorial by loudly talking and laughing. Watch how this soldier quickly puts these people in their place - via the good folks at Conservative Post.