Normally, when you add a card Wagn takes you to viewing that card. This feature lets you take people to another card, or any URL at all. This is particularly useful when people are submitting a form that is not widely visible (which you can do by setting the permissions on that cardtype so that they can create it but not view it).
Here at wagn.org, someone registers a new Wagn by creating a new Registration card, which is only visible to GC Staff. So, they get taken where Registration+*type+*thanks indicates: