The Product Owner is responsible for maintaining the Product Backlog. This includes its ordering in terms of value and effort, its clarity, and identification of what acceptance criteria apply to each Product Backlog Item. It is also very important for the Product Backlog to be ready for each Sprint Planning Meeting so that the team can select the Items for the current Sprint and break those down into tasks. If this is done, the team is able to create an effective Sprint Backlog where it can volunteer for tasks and achieve quick wins each day. If not, the team will likely take on the work of refining the Product Backlog during the Sprint Planning Meeting which is wasteful and not focused. Having a ready Product Backlog helps the team focus during its meetings and ask relevant questions to the Product Owner.
To learn more about Sprint Planning Meetings, visit the Scrum Team Assessment.