The Custom Fields page allows an incredibly detailed view of the Custom Fields in your Jira system. It is important to understand what you can perform on this page, as there is a lot of functionality.
The table itself contains the following columns:
Custom Field ID
Custom Field Name
Custom Field Type
Screen Usage - Screens that contain this Custom Field. You can click on this to see them using the Cross-Reference feature.
Issue Count - Amount of issues that have a value for this Custom Field in the index
Default value set