What's a "Registration"? Is that a Listing?

No. Although it maps one-to-one with listings, it doesn't contain listing data like you'd get via replication from an MLS.

A Registration is the association of a unique listing with the showing vendor application that has authority over it. When a buyer user requests an appointment to show a listing, their application doesn't have to know which other showing platform to send the request to. They send it to the Hub, which gets the seller's application from the Registraton of the listing. The Hub then conveys the details of the request to the seller's application.

This solves the fundamental problem of having multiple showing vendors in a market: they now don't need to write individual integrations amongst themselves. The hub handles the interoperation for them.

Does a registrationId in the listing data means a listing is showable?

No, it means a registration is managed.