The stage where your product meets a real need in a clear market, so customers keep using it, recommend it and pay for it.
Product-market fit means you have found a group of customers with a real problem, and your product solves it in a way that makes them come back, pay and tell others. There is no official moment you reach it, but the signs are clear: demand comes without huge effort, and customers would be upset if the product disappeared.
Before you reach it, the priority is learning, experimenting and adjusting the product, not heavy spending on marketing or expansion. Many projects scale too early and spend on acquiring customers who do not stay. Starting with an MVP is a practical path toward fit.
Practical tip: talk to customers directly and ask how they would feel if the product were no longer available. Watch how many customers come back, not just sign-ups. If people try it and vanish, the problem is usually fit, not marketing.