Product/Market Fit, Defined
Product/market fit means being in a good market with a product that can satisfy that market. It's not a score or a survey result. It's the state where the market is actively pulling your product out of you.
The concept splits the life of any startup into two phases: before product/market fit (Andreessen calls it BPMF) and after product/market fit. These are different companies with different jobs. Almost all startup advice makes sense for one phase and is wrong for the other.
Before PMF, nothing else matters much. After PMF, almost nothing else can kill you.
Key idea
PMF is a good market plus a product that satisfies it. Everything about running a startup depends on which side of that line you're on.