In simple terms, a product backlog is a list of everything that needs to be done in relation to the product. This is the key artifact that helps in translating the high-level product vision and serves as the foundation to initiate the development of the product.
The product backlog includes features to be built, enhancements, fixes, quality attributes, etc. in the form of user stories.
This is beneficial in comparison to the waterfall approach of creating multiple pages of requirement documents because it provides prioritization so that the team has a direction, has the necessary details required by the team, and it keeps evolving and responds to change.