"Fit" is a checklist of subtle details and features that together make up the product. I like to think of it as "When the sum total of nice touches and problems solved that make you love the product is greater than the sum total of annoyances, missing features, and usability snafus that make you fear and loathe the product." Most of these are invisible to someone reading a 1-sentence summary, but you very much notice them as a user.
That's why MP3 players and smart phones failed to get widespread consumer adoption before Apple, why Instagram succeeded where Picwing failed, why Facebook has taken over the world while Xanga is relegated to the dustbin of history, and why Google makes billions while Lycos, Infoseek, Altavista, etc. failed. Same product category, but the product itself was much better.
That's product quality, not market fit. Market fit is reaching the people who would buy your product, and meeting their needs, before you are famous enough for them to find you.
That's why MP3 players and smart phones failed to get widespread consumer adoption before Apple, why Instagram succeeded where Picwing failed, why Facebook has taken over the world while Xanga is relegated to the dustbin of history, and why Google makes billions while Lycos, Infoseek, Altavista, etc. failed. Same product category, but the product itself was much better.