Della wants to buy her husband Jim a nice Christmas present but has only managed to save $1.87 by scrimping all year. The author does not specify what kind of present she has in mind until after she has sold her beautiful long hair and raised another twenty dollars. Then when she has the money to spend, we learn that she has had only one kind of present in mind all along. She goes to many different shops looking for what she wants.
She found it at last. It surely had been made for Jim and no one else. There was no other like it in any of the stores, and she had turned all of them inside out. It was a platinum fob chain simple and chaste in design, properly proclaiming its value by substance alone and not by meretricious ornamentation—as all good things should do. It was even worthy of The Watch. As soon as she saw it she knew that it must be Jim's.
The watch fob chain costs her twenty-one dollars, so she has only eighty-seven cents left. Men did not wear wristwatches in those days. Those who could afford watches carried big, heavy pocket watches, usually in their vest pockets where they would not get scratched up by coins or keys. It was customary to keep a chain and weight (a fob) on the watch, so that if it were accidentally dropped it would not fall all the way to the ground and probably leave both the case and the intricate internal mechanism damaged. Since Jim had an exceptionally good watch, it obviously needed an exceptional chain and fob to go with it.
Everyone knows what happens in the story. Della sells her hair to buy the watch chain and Jim sells the watch to buy expensive combs for Della's hair. The story is sad, but both are comforted by the knowledge that they still have each other.