Usually we make the same reports every week/month and it would be a shame to set it up every time. So once you are satisfied with the report, save it. When you click on save, ManicTime doesn't save the report, just the settings on the left hand side used to generate it, so you can reuse it next week/month. To save the report, click on Save button at the top.
You'll be asked to provide a name. You can also choose to save the date range with the report. If you do, next time you load the report Date range will also be applied.
To load a timesheet, click on the Load button (next to the Save) and choose timesheet to load.