You can infer that Della loves her husband but they are very poor, and she wishes she could provide him something special.
Jim and Della are like many young married couples. They struggle to make ends meet. They are deeply in love, but they do not have much in the way of material possessions. For Christmas, Della wants to find a way to show her husband how much she cares about him. She wants to get him a present. Unfortunately, even though she has been saving for a long time she has not managed much.
One dollar and eighty-seven cents. That was all. … Pennies saved one and two at a time by bulldozing the grocer and the vegetable man and the butcher until one's cheeks burned with the silent imputation of parsimony that such close dealing implied.
Della is upset. She wants something really nice for Jim, something “worthy” of him. She feels like the only thing she can do is “flop down on the shabby little couch and howl.” Yet after Della does that, she comes up with a solution.
Will you buy my hair?" asked Della.
"I buy hair," said Madame. "Take yer hat off and let's have a sight at the looks of it."
Down rippled the brown cascade.
"Twenty dollars," said Madame, lifting the mass with a practised hand.
"Give it to me quick," said Della.
In desperation, Della sells her hair to buy Jim a watchband. She never would have sold her hair, her most prized feature and possession, if Jim did not mean a lot to her. Jim means more to her than physical beauty or vanity. All she wants is to do something nice for him.
The irony, of course, is that Jim sold the watch to buy Della combs for her hair. Yet this demonstrates something. Each knew what the other valued most, and each valued the other more than his or her prized possession. It shows that Jim and Della truly loved each other.