Have you ever approached a door at the same time as someone going in the opposite direction approaches the door? There is usually an awkward moment between the two parties as they try to decide who is going to go through the door first.
This awkward moment doesn’t have to happen if you know the rules of the door.
The person exiting the building or room should proceed through the door or doorway before the person entering comes through.
If you find it difficult to remember this rule, think of an elevator or a bus. You must let the people inside the elevator or bus exit first so there is room for you to enter.