Catalogs are not open for everyone to view as there are some benefits provided by Catalog Owners like price allowance which we charge from them (they pay a partial amount to provide more facility to users). But in order to get these benefits or view the catalog products, you need to have permission by the catalog owner.
Signup code is set by the Catalog Owners to allow users facility to get access right away by just filling this code.
If you have the code, you can enter it here and get permission to view products on this catalog.
Note: Code can be used just once (while registration or logging in if you haven't used it already).