The best way of course to understand and gain an appreciation for a short story is to read it for yourself. This is an excellent short story and it isn't that long either, so it is well worth spending time reading it to gain a sense of what happens. I can only give you an outline, I can't give you how you yourself will emotionally react to this piece of literature, which is just as important as knowing what happens.
Anyway, this classic of Maupassant deals with a woman called Mathilde Loisel, who, it becomes clear, is immensely dissatisfied with her own world and in particular her modest means. She becomes obsessed with daydreams of the wealth that she would like to have and the life that this would give her. One day, she and her husband receive an invitation to a grand party. Mathilde goes to a childhood friend whose circumstances are much better, and borrows a diamond necklace to wear for the occasion. She has an incredible evening and is able to live out her dreams for one night. However, when she and her husband return from the party, they realise that they have lost the necklace. To replace it, she and her husband spend the next ten years working incredibly hard to try and pay back the debt, only to find out at the end that the necklace was not real and they didn't need to ruin their lives.