Product market fit describes the level to which a product sufficiently fills a strong market need. To be blunt, if a product can't fill a need or solve a problem, it's not a good idea. This applies in all facets of business, from consumer products to financial products; the old concept of "the customer is always right" holds true. What's important to remember is that a startup's actions, or lack thereof, can significantly impact its ability to maintain its competitive advantage.
One of the easiest ways to determine if a solution is truly solving a problem worth solving is to ask people. People want to solve problems. If a company develops a solution to a problem that people want to solve, it will likely succeed. There are two ways to look at this approach: Either people are searching for this solution, or the company effectively sells them something and fires an existing alternative in the market.