What is customer discovery? (short version)
This is the first step in the customer development life-cycle.
You need to find who the customers for your product are and whether the problem you believe you are solving is really important to them. Leaving guesswork behind, you should get out of the building to learn what the high-value customer pains are, how your product solves these pains and who specifically is your customer - has the power to make or influence the buying decision - and user - who actually will end up using the product on a repeated basis.
Keep in mind, that this step is not about collecting feature lists from prospective customers, and neither tunning a lot of focus groups. If you're a startup, the founders define the first product. The job of the customer discovery team is to see whether there are customers and a market for that vision.
In this particular step of the process, you should be in the field, listening and discovering how your customers work and what their key problems are.
As action points: you need to dig for negative feedback.
The goal of customer discovery is to turn the founders' initial hypothesis about their market and customers into facts.