A product backlog is a list of tasks that are broken down according to the initial requirements, including all functionality that is functional and non-functional, prioritized by the product owner for the task in the product backlog, and when the development team starts a task, the task is precisely defined and decomposed.
The product backlog is the master of all the features that a product needs to have. When a project is just beginning, no one can foresee all the tasks and requirements beforehand and develop a full, detailed and comprehensive plan for them. The best way to do this is to write down all of the salient features and functions that are required for a project, without a lot of numbers, to ensure that the team's first sprint has a job to do.
As the sprint progresses, producing a release-ready product increment, the customer's intuitive understanding of the product also deepens, and they can suggest changes or additions to the tasks in the product backlog.
At the Sprint planning meeting, the product owner prioritizes the tasks in the product backlog and describes those tasks to the scrum team. The scrum team then determines, based on the overall situation of the team, what functions they will be able to accomplish in this upcoming sprint and move them into the sprint backlog.
What is a product backlog and what is a sprint backlog?