Straddling is the ability of a being or object to exist in more than one plane at any given time. Typically, beings capable of straddling are immensely powerful in a magical sense, and as a result very few have ever successfully managed to do so. The term itself is rarely used in RuneScape, but can be seen most explicitly on the official RuneScape website's article on the Spirit Realm.
The means by which a being is able to exist in multiple dimensions at once remains unclear. Given that the few beings known to have done so are all vastly different in terms of race, nature, and abilities, it seems likely that there are multiple methods of initiating the state.
Examples of straddling
Perhaps the most well known example of straddling can be seen in the behaviour of the Spirit Beast, an ancient being featured prominently in the Spirit of Summer and Summer's End quests. The Spirit Beast, initially confined to a Spirit Realm occupied solely by itself and those it imprisoned there, attempted to enter Gielinor by tearing apart the fabric of reality separating Gielinor from the Spirit Realm. In doing so, it could drain the souls of Gielinor's entire populace, granting it virtually limitless power.
The Balance Elemental, a being created by the deity Guthix to protect the Stone of Jas, is known to exist in multiple dimensions at any one time. This is evidenced by the entry on its RuneScape Duel Card. It is unclear how or why it is capable of doing this, or what dimensions it occupies besides Gielinor.
Solus Dellagar, a powerful mage best-known for his role in the Edgeville Incident in Year 163 of the Fifth Age, is the only known human capable of dimensional straddling. Dellagar, through unknown methods, is capable of slowing the teleportation process to a point that he exists in three places at once: the original point on Gielinor from which he teleported, the point on Gielinor at which he intends to arrive, and in the Abyss that separates the dimensions. This ability is observed first-hand during the Wanted! quest.