Product Backlog is a list of all the activities that need to be done within the project. It is a fine list of requirements that are written by keeping the user as a subject and replaces the traditional requirements specification artifacts. One small piece of the requirement is written in form of a User Story.
These user stories are stored in the product backlog. The product backlog is an important artifact of Agile. The product owner is the primary responsibility to add and prioritize user stories in Product backlog.
In the beginning, Product Owner adds user stories in the product backlog and prioritize them. Product backlog grooming session.