A burndown chart shows the amount of work that has been completed in a sprint and the amount of work that is remaining.
When combined with a velocity estimate, burndown charts can predict if your team is likely to complete the outstanding work in the available time. A burndown chart is also a helpful tool to identify any scope creep, as items will take longer to complete than expected.
Burndown charts are useful because:
You can notice if your team finishes work too early or too late, which shows your sprint planning process needs refinement
If the burndown chart has a sharp drop during a sprint, the work item is likely too complicated