"Custom Fields" are additional properties you want your helpdesk tickets to have. For example, you might want to add a field named "Computer model" to your tickets, so the user fills this field when submitting a new ticket to the helpdesk system.
This page lists all custom fields available in the helpdesk and allows creating new ones. Each custom field has a:
You can rearrange the field order by dragging and dropping.
P.S. You can also add custom fields to Users and Companies within the helpdesk system.