Explanation
The Sprint Backlog is the set of Product Backlog items selected for the Sprint, plus a plan for delivering the product Increment and realizing the Sprint Goal1. The Sprint Backlog is a forecast by the Developers about what functionality will be in the next Increment and the work needed to deliver that functionality into a
"Done" Increment2. The Sprint Backlog is a plan with enough detail that changes in progress can be understood in the Daily Scrum2. The Developers modify the Sprint Backlog throughout the Sprint, and the Sprint Backlog emerges during the Sprint. This emergence occurs as the Developers work through the plan and learn more about the work needed to achieve the Sprint Goal2. The Sprint Backlog is not a commitment to deliver a fixed scope of work, but rather a forecast of what the Developers believe they can do in the Sprint3. Therefore, the Sprint Backlog does not need to be fully defined during the Sprint Planning, but only enough so the Developers can create a realistic forecast of what they can do during the Sprint. References: 1:
Sprint Backlog, 2: The Scrum Guide, 3: Commitment vs. Forecast