Full explanation
NPS comes from one question: how likely are you to recommend us to a friend, from 0 to 10?
People who answer 9 or 10 are promoters. People who answer 7 or 8 are passives. People who answer 0 to 6 are detractors. Your NPS is the percentage of promoters minus the percentage of detractors.
A positive NPS is decent. Above 30 is very good. Above 50 is excellent and a sign that users love your product.
Concrete example
You ask 100 users. 60 give a 9 or 10. 20 give a 7 or 8. 20 give a score below 7. Your NPS is 60 minus 20, which is 40.