Extra Leave can also be defined as Leave in Lieu. Usually, extra leaves are added to a leave management system to add leaves that are non-routine. It can also be used to do manual adjustments either adding or removing leaves.
For example, you may have worked on a weekend, the company may have a policy to give you an extra leave day added to your normal annual leave entitlement or any other leave type. Similarly, extra leave can be added if you work a certain number of overtime hours.
This is completely dependent on company policies, and you should check with the HR department of the organization you work for, for more clarity.