Product discovery and product definition are among a product manager's most important responsibilities. Depending on how well the execution is, discovering the right product and defining it in a way that allows for both speedy and robust execution can be essential to business success.
Product discovery is a process in which many different ideas are generated and then tested with as little effort as possible. “Testing” in this context means validation of whether or not an idea would result in a measurable improvement on a (or more) concrete KPI(s). Ideas that were tested successfully then move forward towards the development cycle.
After discovering product, a product manager needs to define the product. Essentially that means exploring and describing the “What is it?”, “Why do it?”, “Who is it for?” and “How is it done?”. This is of course not only the product managers work, UX/UI design, Engineering, Management, Marketing etc. teams will inform the product definition. The product manager’s job is to provide one concise, reasonable and convincing story (in the literal sense, not as in “user story”) about what the product is going to be.