When your product strongly satisfies market demand
Product-Market Fit (PMF) is the degree to which a product satisfies a strong market demand. Achieving PMF means you've found a product that people love, are willing to pay for, and refer others to. Marc Andreessen called it the only thing that matters.
You know you have PMF when you can't keep up with inbound demand, NPS is 50+, and customers are upset when you consider removing features.