The “Windows.old” folder is a kind of backup copy of an older version of Windows. It contains the various files and settings from the previous installation of the Microsoft system which are necessary to restore the old version. Normally, Windows.old is automatically deleted after a specified period (ten days), but in practice this does not always happen leaving the folder, which is often several gigabytes in size, on the system partition alongside the current Windows folder.
You might encounter such a scenario when you perform a new Windows installation on a hard drive where Windows (the same or a different version) is already installed. In this case, the Windows.old folder is often generated but not always automatically deleted by the Disk Cleanup tool. In some cases, the backup folder will remain on the system after the specified storage period has passed following a system upgrade (e.g. from Windows 8 to Windows 10) or a Windows version update.