The multiverse is the greater collection of universes that together encompass everything that physically exists. The multiverse is comprised of an unknown number of much smaller universes, whose physical existences are separated in a variety of physical and magical ways.
In theory, the whole of the multiverse is filled with The Void, a lack of existence that, to an inhabitant, would appear to be nothing but an infinite field of white emptiness. The isolated universes within this Void contain far more complex physical laws than the Void itself, including space, time, matter, and energy. Although it is possible for matter to enter the Void and retain its physical existence, there is nothing known to inhabit the Void by nature. These worlds are often, although not always, separated by the Abyss, a border dimension that helps to the worlds to maintain their physical selves.
The world of Gielinor, in which the majority of RuneScape's actions take place, is but one of the universes within the multiverse. Players are able to visit a number of other worlds outside Gielinor, but it is impossible for them to leave the Multiverse by its very definition.
The only in-game occurrence of the term multiverse is seen when players speak to star sprites, cross-dimensional travellers who take on a different physical shape when they enter Gielinor.