Product Owner person whom holds the vision for the product and is responsible for maintaining, prioritizing and updating the Product Backlog. In Scrum, the Product Owner has final authority representing the Stakeholder's interest in Backlog prioritization and requirements questions. This person must be available to the Team at any time, but especially during the Sprint Planning meeting and the Sprint Review meeting.
Challenges of being a Product Owner:
Resisting the temptation to "manage" the Team. The Team may not Self-Organize in the way you would expect it to. This is especially challenging if some Team Members request your intervention with issues the Team should sort out for itself.
Resisting the temptation to add more important work after a Sprint is already in progress.
Being willing to make hard choices during the Sprint Planning meeting.
Balancing the interests of competing Stakeholders.
The Product Owner is the keeper of the requirements. He provides the “single source of truth” for the Team regarding requirements and their planned order of implementation. In practice, the Product Owner is the interface between the business, the customers, and their product related needs on one side, and the Team on the other. He buffers the Team from feature and bug-fix requests that come from many sources, and is the single point of contact for all questions about product requirements. He works closely with the Team to define the user-facing and technical requirements, to document the requirements as needed, and to determine the order of their implementation. He maintains the Product Backlog (which is the repository for all of this information), keeping it up to date and at the level of detail and quality the Team requires. The Product Owner also sets the schedule for releasing completed work to customers, and makes the final call as to whether implementations have the features and quality required for release.