The Availability approvals setting allows Managers and Admins to approve/decline an Employee's Availability submission.
This setting is managed under Company Settings > Availability:
(NOTE: Only Admins can access the Company Settings)
How it works:
Here's an example of what an Employee would see for their Availability on the mobile app:
The question mark denotes a pending request, and red 'x' is a declined request. All the other entries have been approved.
If you decline with a comment, Employees will see this:
The Employee can then make the necessary modifications before re-submitting for approval.