Burndown chart helps teams understand the amount of work that has been completed and the remaining work in a sprint. It also helps you understand how much ad-hoc work has been pulled into a sprint after starting it.
Insights that you can derive from the burndown chart
1. A spike in the number of tasks in the middle of a sprint relates to the amount of ad-hoc work that is being pulled in.
2. During the sprint, if there's a spike in story points without any ad-hoc tasks, then it shows that tasks are being re-estimated. This signifies that your team needs to get better at estimates during planning.
3. If your team is consistently finishing tasks much earlier than anticipated, it could be an early that your team is under committing and there's room for more tasks.
Ideal line vs Actual line
The idea line (grey) represents a linear burndown of tasks/story points in the sprint and if the actual line (green) is mostly below ideal, then it indicates that chances of completing all tasks in the sprint is more.