Preview mode is for reviewing all the content Redact targets according to your rules. This will not delete any items, it will just show you what items are being queried.
Deletion mode will remove all the contents queried according to your rules. The items that get deleted will not be recoverable. You are able to pause, continue, or cancel the deletion process, but you cannot un-delete something.
The Scheduler allows you specify what time and date your deletion will run, in addition to allowing you to repeat the removal every X days. When you input all your preferences, Redact will automatically provide you with a small preview that shows when the next 3 scheduled deletions will occur.