The Product Owner is typically responsible for collaborating with stakeholders, users, and customers to understand their needs and expectations, and to validate the value of the product. The Product Owner is also responsible for working with the Scrum Team on Product Backlog refinement, which is the act of adding detail, estimates, and order to Product Backlog items.1 These two responsibilities help the Product Owner to maximize the value of the product and the work of the Developers.
Reference:
1: The Scrum Guide2, page 18
2: The Scrum Guide