Do you have a problem worth solving? This is the question behind problem/solution-fit. Look at an existing problem and try to find a solution that fits the problem. Keep in mind that not every solution suits the problem. A solution might counteract a problem but not solve it at all.

The Basics

A solution which does not suit the problem is one of the reasons startups often fail. So be aware of the specific problem and work closely with potential customers to create a viable product. Take for example the idea of festival organisers. They address the problem of festival visitors who quickly run out of battery on their phones. Due to a lack of electricity at festivals it is not possible for them to charge their phones. The solution is simple. The festival provides a booth on the terrain where the visitors can leave their phones and charge them for a minimal fee. This solution sure solves the problem but does it solve the main use case? Would it not be better to hand out small portable power banks for a deposit? The idea of a charging booth has been in place but it soon was removed by the power banks. The transformation happened simply by asking customers about their experience.
Steve Blank describes in his book The Startup Owner’s Manual problem/solution-fit as part of the Customer Discovery process which is the first step. This process includes a development of a set of hypotheses and the implementation of tests for these hypotheses via customer interviews and landing pages. After this phase the question becomes do you have a solution which customers want to pay for? And with this question you have reached the stage of product/market-fit.

Find here a short explanation of problem/solution-fit with an example from Daniel Bartel.
This image shows the position of problem/solution-fit within the three stages of a startup.