A flash sideways is a version of the survivors lives had they never crashed on the island and/or had Jacob not intervened in their lives. Presumably it would be their life if the island and none of it's inhabitants ever existed. Think of it like a fork in the road. The road being the original timeline. The original time line meets the fork and splits, original timeline takes the left fork and the flash sideways timeline (if there never was an island) takes the right fork. This timeline was created when Juliet detonated Jughead.