As described in the Scrum Guide, the Sprint Backlog contains the set of Product Backlog items (PBI) selected for the Sprint, plus a plan for delivering the product Increment and realizing the Sprint Goal.
In order to manage its progress during the Sprint, the Development Team needs to have a clear understanding of the work remaining to deliver the product increment and achieve the Sprint Goal. During the Sprint planning the Scrum Team creates a work forecast for everything that the team plans to accomplish during the Sprint. In order to understand better whether the team can complete a PBI in the Sprint, the team usually subdivides it into Sprint Backlog Items. These items may represent tasks that the team must complete, or any other unit of work that helps the team understand how to achieve the Sprint Goal within the Sprint.
Sprint Backlog is not a fixed list of work to be done. It is a transparent, flexible, living plan by the Development Team on how it will meet the Sprint Goal. It enables the Development Team & the Product Owner to discuss the work in the Sprint Backlog during the Sprint and to make adjustments as needed.
To ensure continuous improvement, the Sprint Backlog also includes at least one high priority process improvement identified in the previous Sprint Retrospective.