In burn down, chart X-axis shows working days and Y-axis shows remaining efforts.
Here, in the image blue line depicts the ideal time suggested for completing the project. For example in a project ideally, 28 tasks (efforts) are pending on the starting day of the project. And the project is scheduled to be completed in 20 days. So on the 20th day, zero tasks (efforts) should remain, which means the project should get completed.
On the other hand, the red line depicts the actual task (efforts) given with respect to the days. Here on day 5, 7 tasks need to be completed as per planning, however, in actual only 4 tasks were completed.
Thus, it provides proper tracking down of the progress of the project and thereby helps in improving the efficiency and on-time taken to deliver the project.