Time off approvals allow you to set which users can approve employee time off requests, and in what order. It can be accesses from the left menu under Users - Time Off Approval.
On the left you can select the user to edit the settings for:
On the right side you can choose if they have access to approve (Note: their user role must also have access to the time off requests screen, see this article for more details on user roles). Then you can set the following settings: