Possible meanings are 1) "in the region" as Jesus is at the sea in the region of the Decapolis or 2) "through the region" as Jesus went through the region of the Decapolis to get to the sea.
This is the name of a region that means Ten Cities. It is located to the southeast of the Sea of Galilee. See how you translated this in [Mark 5:20](../05/20.md). (See: [[rc://en/ta/man/jit/translate-names]])