Severed rooms are rooms that do not have a choice that links to them. This usually happens when somebody deletes a room that had choices leading off of it. This makes those rooms severed, since they no longer have rooms linking to them through choices. A funny side effect of this effect is that the first room of a story is almost always considered a Severed Room since (unless you link another room back to it) it has no choices leading to it from any rooms.

You can see a list of all severed rooms in a story you wrote by selecting the "Severed Rooms" link in the "Administration" section of the story sidebar. This brings up a list of all the Severed Rooms in your story and gives you a link to each of them.