A business idea is only valuable if it solves a real problem that people will pay to have solved -- 'problem-solution fit' means confirming the problem exists and matters before designing a solution. Customer discovery interviews involve talking directly to potential customers about their current frustrations and workarounds, without pitching your idea, so you hear honest, unbiased information rather than polite agreement. A minimum viable product (MVP) is the simplest version of your offering that lets you test a core assumption with real customers -- this could be a single product variant, a manual service before building an app, or even a landing page that measures interest before anything is built. Validating early, even informally with 10-20 conversations, is far cheaper than discovering after months of building that nobody wants to pay for the solution.