A product backlog tracks what the product team works on. These are the larger goals the Product Owner wants to achieve. The Sprint Backlog > User Stories > Tasks are created from the Product Backlog and decomposed into the next smaller piece for work.
Depending on the size of your organization, you may have one central Product Backlog or multiple product backlogs for different teams. The Product Owner will periodically refine (aka groom) the Product Backlog to ensure the most important initiatives are at the top. Each initiative has all of the information needed to execute against it. The product backlog feeds the sprint backlog and, eventually, the user stories.