Ask HN: What is Product-Market Fit? Explain like I'm five

Just saw a guy on Twitter claim that he grew his client's revenue from $170k to $850k, because of product-market fit. (https://twitter.com/zachmstuck/status/1293570074054397952)

I was under the impression that product-market fit means how well your business idea meets the market demand. Well, how can then a consultant change that? Am I missing something?