This phrase was coined by Maurya to describe the point in Customer Development where you have established market evidence that the solution you propose will solve a customer problem.
Problem-solution fit starts as an assumption: “I will create product a, with functionality b, that will satisfactorily solve market segment c’s problem d.”
Without actually producing product a, you use the methods covered in this book to generate evidence that a with b will indeed solve d for c. You choose to progress in developing your idea only when the evidence provides you the confidence to do so. Confidence is, of course, highly subjective. But when you have achieved this point of confidence, you have problem-solution fit.
Problem-solution fit answers the question: “What evidence do I have that reduces the chance I’m wrong sufficiently to warrant the investment of time and money to take the next step?”