Product market fit means being in a good market with a product that can satisfy that market. Getting there is the hardest challenge your start-up faces. In idea validation and the process of finding product-market fit trial and error is crucial. But the goal for most startups is to launch with a validated idea that customers immediately flock to. This can be done with a continuous cycle of customer feedback. Much like the lean startup "build, measure, learn" approach, I like to think of it as the three I's - ideas, input, and iteration.