In this phase you will test whether your innovation and your business concept actually is the right solution for a real problem - and whether the concept has business potential.
Nothing beats speaking to real live customers - or people in the industry...
Understand the problem from the user/customer's perspective - and find out what is the actual problem...
(Actually it's better to do that in Step 1: Empathize)
Keep track of what you want to test, and what you learned from your testing...
Testing is not just about the product. It is also about testing whether your business model will live up to your assumptions in real life...
Turn your business model assumptions into a budget that simulates what the bottom line will look like if your assumptions are true.
Part of testing your business idea and business model is also identifying and evaluating the potential risks - what could go wrong - and finding ways to mitigate those risks.