In Scrum methodology, term Backlog is a prioritized list that contains short description of the functionality required in the project.
The stakeholders want to ensure they get what they need from your project. A key aspect to keeping the right priorities throughout the project is the on-going refinement of the product backlog. This means that the PO is out there meeting with stakeholders to understand what's needed. They're constantly moving the work around as the business needs and requirements shift.
Throughout the project, your backlog is constantly changing as new features and stories are being found and added. Other stories are being changed and maybe even removed.
As new stories are discovered, the Product Owner works out the details and acceptance criteria for each one. But at least once per sprint, the whole team must come together to see what new items have come up. This is usually a 30 to 60 minute Backlog Grooming Session. It usually happens around the midpoint of the sprint. The Product Owner reads the new stories to the team, and the team asks about any details that are missing.
The Product Owner for the scrum team is like the driver for a vehicle. For the verhicle to run smoothly and complete its journey, the driver must work well and provide anything that the vehicle requires, be it fuel or repair.