It could just be that the people who live there consider the continent to be the middle of the world. There's precedent for other civilizations considering themselves to be the middle or center of the world.
But it could also be metaphysically between things. Between the formless heavens of divinity and the mortal world. Between life and death. Etc.
I think either of these two ideas (btwn life and death, btwn mortality and divinity) are likely, given how much the culture we see in the game has to do with death and burial. And the world of the game is one physically inhabited by gods and demigods AND in which seemingly disembodied divine forces (Outer Gods) exert powerful influence.1 It's also a place where the laws of reality are directly accessible and malleable by way of the Elden Ring.
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Except, some have suggested that "Outer God" might be better understood as a divine force that exists outside the Golden Order of the Erdtree. The nameless god of death is referred to as an Outer God, likely because the Death Rune was removed from the Elden Ring. But it seems to have been very important to the burial rites of previous ages, so some time before Marika's Golden Order, perhaps it wasn't an Outer God at all.