Startups live and die by their product-market fit (PMF). Everyone talks about it as the Holy Grail, but it can be surprisingly hard to define: the best descriptions out there boil down to "you'll know it when you hit it." That's not very helpful.
Let's backtrack. What are startups? One definition: a company created to find and grow new opportunities, usually under high risk and uncertainty.
At the beginning, you're in exploration mode. You might have an idea for a product or an insight into a problem that needs addressing.
However, that's just a hunch. The early days are all about trying to prove your hunch is correct or, if not, finding a better idea. The only thing that matters is finding a market that truly wants what you're building, or vice versa: finding what you need to build for a market you've identified.
Paul Graham's "make something people want" and Steve Blank's "customer discovery" are both about this: search, test, iterate.