Free (or unused) space on disks may contain some remains of data that you want to keep private. When wiping the free space,  overwrites it with a predefined data pattern rendering it impossible to recover the overwritten data. You may read more about those patterns in the Wiping Algorithms help page.

To select disks on which their free space will be wiped,

The Info panel will show wiping options for this operation. You may change them on the Options panel.

If the disk wiping has been abnormally interrupted, some temporary files and folders may remain on the disk consuming its space. Upon next startup, SecretBurner will show the warning message asking you if you want to remove those temporary folders and files.