So Della want to get a nice Christmas present for Jim (A watch chain thing). But the thing she knows he wants, costs 20$, while she only has around 1 dollar. Della decides to cut her long hair, and sell it so she can get a present for Jim.
The irony of it though, is later, Della comes home with short hair, and Jim's present, when Jim comes home as well.
Jim sold his watch to get beautiful combs that Della wanted. Meaning that not only would Della not be able to use the combs, but Jim wouldn't be able to use the watch. So they both put the presents away, and resolve to be happy with what they have.
Hope this helps!:)
