NPS (Net Promoter Score) is basically a quick way for an online store to see how happy its customers are. It comes from asking one simple question: "Would you recommend us to a friend?" People rate it from 0 to 10. If they give a high score, they're fans (promoters); low score, not so much (detractors). The higher the NPS, the more likely people love the store and will tell others about it.