The Ghostly Plane is a desolate and empty plane somewhere between life and death. Spirits of the deceased are taken here by the Angel of Death before moving on to the afterlife. In this plane, there is no magic.
As part of his plan to kill Wyatt, Gideon ordered the Darklighter Damien to trap Piper Halliwell and Leo Wyatt in the ghostly plane. While trapped here, they were hunted down by the Darklighter and eventually conceived Chris Halliwell here before being rescued by her sisters.