This is the page that opens when you use the Lookup button next to Anchor field
Definition Anchor fields :
Existing reference fields on the selected page.
Where do we want the new field positioned on the page :
Addafter: The new field will be positioned on the page after the anchorfield
Addbefore: The new field will be positioned on the page before the anchorfield.
Tooltip :
Create your own tooltip for the user how to use the new defined field.