The fantasy world of Glorantha is commonly described in terms of 3 planes of existence: the Mortal Plane, the Spirit Plane, and the God Plane.
In fantasy gaming as in fantasy literature, interplanar travel ("world hopping") is a staple of the genre. World hopping provides a plausible means to introduce non-Gloranthan creatures, characters, and magic to the campaign. To facilitate this, many campaigns also include a fourth plane, the Hero Plane, somewhere between the Spirit and God planes in terms of time-binding. Typically, the character(s) appeal to a priest or invoke unpredictable magic to open a Hero Path. Between the Mundane Plane and the Hero Plane, they must face a challenger, usually a cultist or creature aligned with an opposing Rune. (Sometimes this is said to take place in the Plane of Conflict.) If they defeat the opponent(s), they may pass to the Hero Plane (that is, another game world). Some GMs follow an elaborate system of game mechanics to integrate this capability into the campaign; conceivably, a character who mastered the necessary spells would be able to access the Hero Plane more or less at will. Other referees prefer to bind this ability strictly to powerful NPCs or plot device events, locations, and/or magical items. RuneQuest enthusiasts refer to this aspect of play as "Hero Questing"; unfortunately, the name "HeroQuest" was squatted by another game company.