The Sprint Backlog represents the work that a Development Team needs to pull from the Product Backlog to achieve the Sprint Goal. The Sprint Goal is an objective set by the Scrum Team during Sprint Planning and captures the hypothesis that the team wants to test, a goal it wants to achieve, or an experiment to run. Although the Sprint Goal is fixed during the Sprint, the Sprint Backlog is not. This would be unwise considering the core premise of Scrum: we can't create detailed plans for the future. Even if that future is a single Sprint, it is entirely possible that new insights or impediments emerge as the Development Team works through the Sprint Backlog.
A team might learn that a technology they picked does not perform as expected. Or a key feature needed to reach the Sprint Goal was missed during the Sprint Planning. As issues emerge, changes to the Sprint Backlog may be warranted in order to reach the Sprint Goal. The Development Team will then re-negotiate the Sprint Backlog with the Product Owner. In short; a Sprint Backlog is flexible, as long as changes do not distract from the focus on the Sprint Goal.
The Daily Scrum presents Development Teams with an excellent opportunity to inspect and adapt upon their progress to the Sprint Goal and make adjustments to the Sprint Backlog when deemed necessary.