Although it may not look like there is much going on, the process of guarding the Tomb and changing the guard is very intricate. The Sentinel on duty takes 21 steps, then turns to face the Tomb for 21 seconds. The Sentinel then turns again and is facing back in the direction from which they just came, moves their rifle to the shoulder away from the Tomb, and waits another 21 seconds. They will then take 21 steps and repeat the process for the duration of their shift. Depending on the time of day and the weather their shift will last 30 minutes, one hour, or two hours. Each shift will normally end at either the top of the hour or the bottom of the hour but could end at another time if circumstances require it.
After each shift, the guard is changed. This ceremony is initiated by the Sergeant of the Guard exiting the Tomb Guard Quarters accompanied by the new Sentinel. Their appearance in the archway near the south end of the plaza signals the Sentinel on the track that they are being relieved.
As the Sentinel makes his march to the north end of the track, the Sergeant of the Guard enters the plaza and waits for the Sentinel to turn back to the south. The new Sentinel will then enter the plaza and turn toward the inspection station. Each Sentinel and the Sergeant of the Guard will then take 21 steps to their next position in the ceremony.
After addressing the audience, the Sergeant of the Guard will march to the inspection station to verify that the incoming Sentinel has a clean rifle and that their uniform is perfect before allowing them to guard the Tomb. After the inspection is complete, each member will march 21 steps forward. Orders are then passed from the outgoing Sentinel to the incoming Sentinel, the new Sentinel is posted, and the Sergeant of the Guard escorts the outgoing Sentinel back to their quarters.
You may have noticed that the number 21 is a common theme. The 21-gun salute is the highest honor paid to an American service member. In recognition of that, as many components of the ceremony as possible are designed around No. 21.