The goal is to formalize the time submission process, and prevent users from editing their time sheets after they have been processed by management and/or finance.
There are two options: with users formally submitting their log, and without. Choose with if you like to give more control to users when they consider their logs final - they will be able to submit them from their calendar view. Choose without when admins can approve (and thus lock) any time log at any time.
When you turn on approval, all previously entered entries will be considered approved and thus not editable. All new entries will be created in draft mode and still editable until approved/submitted.
As an admin you can (un)approve entries from your reporting page looking at the logbook grouping option. There, you will have the option to select specific entries and approve them using the drop down 'for each selected row...'. With the approval enabled, you can also filter to only see the submitted/approved/pending entries.
Please be aware that you can also enable this feature for just a single business unit. This enables greater control for admins, but also allows to test with a few users before enabling it for all.