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.
The Old Guard sentinel lets them know just how seriously he takes his job — and the preservation of dignity for the Tomb of the Unknown Soldier.