Custom fields are used to customize products. This can be just product specifications just listed the same way, or really different attributes of a product which triggers different shop actions. For example an attached downloadable good, or adding to a shoppergroup.
This view lists the custom field prototypes. The custom field prototype sets the basic configuration and also tells VirtueMart that there is a custom field. These prototypes are then listed in the product edit page and configured for the product itself in the product edit custom field tab. Be aware that related categories and related products are also customfield. You can edit them, but never delete them else you may get in trouble.
More details about the different customfield types docs.virtuemart.net/manual/general-concepts/212-customfields.html