It's a symbol of love and sacrifice. Della really wants to give Jim a present that will go with his gold watch which was a prized possession to Jim. And Jim wanted to get Della a comb for her beautiful hair. However, they both didn't have money so they made a sacrifice to get each other a present. Della cut off her beautiful hair to get Jim the chain, and Jim got rid of his watch to buy her the comb. Even though, the gifts are now worthless it shows how they thought of each other and made a sacrifice to get each other a gift.
