Pre-Problem-Solution Fit, you concentrate on learning as much as you can about the problem, who are the real customers (user? buyer? boss?), and possible solutions.
Pre-Minimum Viable Product, you concentrate of learning, developing and testing the minimum features and functionality required o solve the problem to a degree the customer will buy.
Pre-Product-Market Fit, you concentrate on learning about funnels, testing messaging and positioning, and likely iterating on product and market segment in search of P-M fit.