What is "Autosave" feature?

This feature automatically saves the content of VFB in Corona EXR format during rendering based on given interval, so that it can be resumed later in case of system crash, overheating, or other problem. You can enable it under Render Setup > System > System Settings > Output autosave. Keep in mind that autosaving a high-resolution image frequently may have negative impact on rendering performance.
Also see: How to pause rendering and resume it later?