This is where you can get from one explorable area to another. An area boundary is represented by a glimmering curtain of white light through which you can see the adjacent area. Walking past this curtain results in the next explorable area being loaded. If one party member enters a new area, the whole party will automatically follow him. This can be uses to bring back lost or dead party members (the dead will be resurrected). Although going back and forth between areas is usually not the preferred method of resurrection, because if you re-enter an explorable area, all mobs will be reset to their initial position (and you will have to kill all that you previously may have killed again).
On the map area boundaries are marked as small white/bluish vortexes.
Most area boundaries have a Resurrection Shrine nearby.