The Add Filter option allows you to filter various Lists using Custom Fields you've created under that are either a Checkbox, a Pick-list, or a Date based field

The Custom Field can only be filtered by the relevant list. Eg a Booking Custom Field can only be filtered on the Booking List.

You cannot search using the following Field Types: Text (both Single & Multi Line), Number, HTML or Currency.

When filters are not available due to the Custom Field Type, you can instead use customized columns to view the relevant data: