Overworld is a psychic plane of existance that telepaths can go to. In Overworld space, time and distance have no meaning and one can easily get lost and lose track of time and never return to ones body.
A Tower circle will travel to Overworld to do psychic combat with other Towers during the Ages of Chaos. Due to the Compact this form of combat is no longer used, the one exception was when the circle of Arilinn under Lady Leonie Keeper of Arilinn attacked the "Forbidden Tower".
Many times when a telepath or circle travel to Overworld they will create psychic markers such as creating a psychic version of their Tower or some other landmark. If a telepath is lost in Overworld without a marker only the call of another telepath or telepaths can usually bring them back.
When in Overworld many telepaths have been known to see deceased loved ones or phantom shadows that can lead them astray.