He [Abraham] was sitting at the entrance of the tent as the day grew hot... Genesis 18:1
The entrance to the tent is covered by a curtain which hung down from the top of the entrance. The Hebrew word "dal" means "hang down" and is the root for the word "dalet" meaning "door.” This word is also the name of another Hebrew letter, a representation of the tent door.
The door of the tent is the most important part of the tent, not because of its appearance, but is function as the entrance into the tent. The door of the tent can be equated with the throne of a king.
In the Hebrew culture, the father of the family is the "king", the one who holds full authority over the family. The father will often sit at his door much like a king will sit on a throne. All family legal matters were performed at the tent. Here he will watch over his household as well as watch for passing travelers. The nomadic rules of hospitality are very strict and complex.