In agile development the work to be done is kept in a prioritized list. The product backlog is that list. It contains user stories, bug fixes, technical tasks, and other items that the team may work on. The product owner is responsible for ordering the backlog so that the most valuable items are at the top.

The backlog is living: items are added, removed, refined, and reordered as learning occurs. Higher-priority items are broken down into enough detail that the team can estimate and complete them in a sprint. Lower-priority items may remain large and vague until they rise in importance.

A well-maintained product backlog gives the team and stakeholders a transparent view of what is planned and what is not. It is the single source of demand for the development team. Refinement sessions keep it healthy so that sprint planning can focus on the near-term work rather than on rediscovering priorities.