Complex sentence: When activated, Returnil virtualizes the system partition (the partition or volume that the Windows system files are stored and booted from), and all changes made are written and stored in a cache, essentially sandboxing the entire system partition.
Split into simpler sentences: When activated, Returnil virtualizes the system partition (the partition or volume that the Windows system files are stored and booted from). All changes made are written and stored in a cache, essentially sandboxing the entire system partition.