The Sprint Backlog is one of the 3 artifacts in Scrum. It represents a subset of the Product Backlog and is formed during Sprint Planning.
Sprints in Scrum are timeboxed iterations in which Scrum teams work to deliver incremental value to their customers. Typically lasting 2 weeks in duration, the Scrum team decides what they believe they can achieve in that time. This work forms what is known as the sprint backlog.
The Sprint Backlog is formed during Sprint Planning. During Sprint Planning Scrum teams will set their goal for the Sprint known as the Sprint Goal. The Sprint Backlog is then formed by pulling in backlog items from the Product Backlog that will be completed in the current Sprint to achieve the chosen Sprint Goal.
The Sprint Backlog is a real-time view of what the team is working on right now and how they intend to achieve the Sprint Goal and deliver the next increment of value to their customers and the business. Like the Product Backlog it is also emergent. The Sprint Backlog can change during the Sprint where necessary to achieve the Sprint Goal. The team refines their Sprint Backlog each day during the Daily Scrum where they discuss their progress, raise impediments to the Sprint and make necessary changes.