The Tomb of the Unknowns is located in Arlington National Cemetery in Virginia and is a monument dedicated to American military members who died without their remains being identified. According to CNN :
The tomb guards are soldiers of the United States Army’s 3rd Infantry Regiment. It is considered one of the highest honors to serve as a Sentinel at the Tomb of the Unknowns.
Fewer than 20 percent of all volunteers are accepted for training and of those only a fraction pass training to become full-fledged Tomb Guards. This attrition rate has made the Tomb of the Unknown Soldier Guard Identification Badge the second least-awarded decoration of the United States military.
The soldier “walking the mat” does not wear rank insignia, so as not to outrank the Unknowns, whatever their ranks may have been. Non-commissioned officers (usually the Relief Commander and Assistant Relief Commanders), do wear insignia of their rank when changing the guard only. They have a separate uniform (without rank) that is worn when they actually guard the Unknowns or are “Posted”.