A geo is a region of the game world, a number of areas grouped together because of geographical proximity. These areas are usually only accessible via a large and mostly open area that shares its name with the geo. The exception to this rule is the Void Geo, which is used for the left-over and tricky areas like Limbo, Asgard Nexus and Midway of Despair.
Geos have two purposes: the first is aesthetic as they help to break up the large continent that makes up most of the Realms and to typify areas of the geography (like how the Desert of Despair Geo makes up all the desert-themed areas); and the second is the role they play in astral disruption's limitation of planar spells.
You can check which geo you're in with the 'geo' command.
Raven Tor is part of the Forgotten Woods Geo and is only accessible via The Forgotten Woods area.