This service lets you resolve IVOA identifiers, i.e., URIs referencing Virtual Observatory resources (they have a URI scheme of ivo://) to the resource records. There you will find all kinds of metadata there, including access options (“capabilities”).
To do this resolution, take an IVOID and prepend it with https://dc.g-vo.org/I/.
For instance, to this service is registered as ivo://org.gavo.dc/rr/q/nmah. To see the registry record, you would paste https://dc.g-vo.org/I/ivo://org.gavo.dc/rr/q/nmah into your web browser's address line.