An address that exists without being advertised
Every published page gets a URL, and that URL does not put the page anywhere. There is no directory of webinar pages to browse, no listing your session appears in, no shelf of events a stranger can walk past. A private webinar page joins the world exactly as far as you share its link, and no further. The invitation is the distribution list; that is the whole mechanism.
This is different from locking the page with a password, though the two work well together. An unlisted page is quiet rather than closed: anyone holding the address can open it, but the only people holding it are the ones you chose. For most closed sessions, being unreceived is what being private actually means.