A doorway is an entry page for a free site that isn't the site's true index. You see them named index1.html, index2.html, and so on. Sometimes they are named with a keyword rather than numbering them. If you put both an index.htm and an index.html in a free site's directory, both can be considered doorway pages.
It's just a thing submitters do so they can submit to every link list known to man. But it's horribly unpopular amongst most link list owners.
EDIT: That damned Chris guy replied while I was typing.
My reply is just more frees site specific.