Product-market fit is when a product solves a problem so well for a specific group of people that they keep using it, pay for it and tell others about it without being pushed.
It shows up in behaviour more than surveys: users come back on their own, churn is low, growth comes from word of mouth, and people are upset when the product is down.
Fit is with a market, not with everyone. A product can have strong fit with freelance designers and none with large agencies.
Why it matters for a launch
Before fit, the job is learning and changing the product. After it, the job is growth. Spending heavily on marketing before fit usually brings users who leave.