It will display the Burndown chart for your currently running Sprint. To see the Burndown chart for a Sprint, you need to start the Sprint first in the Plan mode.
You can view the Burndown chart for active Sprint. You can also see the chart for the completed Sprint.
Looking at earlier Sprint Burndown charts, you can compare how the team is performing over releases in term of completing work in a Sprint. As discussed in the earlier section based on the nature of typical Burndown charts, you will be able to compare Burndown charts for different Sprints and take relevant next steps to improve upon how work is completed in a Sprint.