Jim and Della have a very close, loving relationship. And with Christmas just around the corner they want to express their love through the exchange of gifts. Jim buys Della some combs for her pretty hair, and Della buys Jim a chain for his pocket-watch. Unfortunately, Jim and Della are dirt poor and don't have a lot of money to spend on gifts. The only way that Della can afford to buy Jim's gift is by selling some of her beautiful locks to a fancy wig-maker. And the only way that Jim can afford to buy Della's gift is by selling his pocket-watch.
Both Jim and Della have ended up with what are, from a practical standpoint, worthless gifts. But they're so much more than that. The combs and the watch-chain are expressions of the deep love that Jim and Della have for each other. It makes no difference that they have no practical use-value; what matters is the loving thoughtfulness that went into choosing each gift.