Every entrepreneurs you have to ask about why you founded a startup companies, You will receive answers like: "I wanted to make the world a better place." but more often, the simple answer like "I saw a problem and found a solution to it." Entrepreneurship, at its all about core, it is solving problems.
A problem slide you have to describe how your product will solve the market problems and how to will tackle current problems. The easiest way to explain the problems to tell a short summary about a person that experienced the problem in real world and why the person needs a solution to it.