Project Backlog in Scrum is a dynamic document which includes all tasks or goals that are necessary for product development or release. It is an ever-evolving list of user stories (all details related to a particular feature or task) and tasks to be executed or achieved by the scrum team.
The Project Backlog is maintained and reviewed by the product owner who is responsible for prioritizing the user stories or tasks and setting clear scope and objectives. The scrum team is then responsible for accomplishing the user stories in the backlog as part of their sprints.
The goals of the Project Backlog are:
• To define, prioritize, and estimate the work to be completed
• To track progress during a sprint
• To provide visibility on user stories and associated tasks
• To provide a top level list of all stories, tasks, and features to be implemented
The Scrum team should maintain the backlog by regularly re-prioritizing, creating story estimates, and adding new stories to the project backlog as they become known.
Here is a useful article that explains further on Project Backlog in Scrum: