Customer development is when founders cold email (or Tweet, whatever is appropriate) and talk to customers trying to test the following in the following orders:

1. Is the problem that you're going to solve important? Do people respond to cold email about a hypothesis to solve it? (testing problem relevance)

2. Is the solution that you put forward realistic sounding to customers? Do they believe that you'll be able to build it, and want it if you do? (testing solution relevance)

3. When you show customers an early version of what you're building, do they ask to use it (testing problem/product fit)

4. When early users get access to your sketchy prototype, do they usage retain? Regardless of funds changing hands, do they keep using it?

If you have a product where you can email people about a problem, who consistently reply, see a demo of your product and ask to use it, then adopt it and continue using it without pressure, you should figure out pricing.