The Progress chart shows you the actual and planned percent complete of tasks within the project project. In short, it tells you how much of the project has been completed to date, vs. how much you expected to be complete by today.
Actual Progress - Top Bar
The actual progress bar shows the summarized percent complete of all tasks in your project. For instance if half your tasks are 0% complete and the other half are 100% complete, then it will show the actual progress as 50% complete. To update the Actual % complete, then open the Project page and change the % complete for each task.
Planned Progress - Bottom Bar
The planned progress bar shows the percent complete that your project should be today. For example, If you have 2 tasks of the same duration and one was due to be 100% complete by today and the other wasn't due to start until tomorrow, then overall, your planned percent complete would be 50%. To reduce your planned percent complete, update the due dates of tasks in your project so that fewer tasks are planned to be completed by today. This bar is not displayed by default. To add it, click the settings icon on the chart and select "Show Planned".