It lies at the edge of the known universe, in the Promethean Galaxy, built up by the bodies of all who have tried to pass it, vainly attempting to see The Source, which lies behind it. It is theoretically permeable but countless have tried and failed, from all over the universe.

Now that the DC Universe is revealed to be 52 universes it is stated that each reality has a Source Wall separating it from the others with The Bleed in between them. Superman-Prime managed to destroy a Source Wall causing the destruction of Earth-15.

The Monitors caution that passing between realities breaks down a Source Wall and eventually other Earths could end up like Earth-15.

Also in the Death of the New Gods series it is seen that the New Gods are no longer passing on to the Source and instead they are forming a second Source Wall as the God-Killer is stealing their souls.

Some have managed to pass the Final Barrier and entered the Source, all viewing it differently:

Lucifer: A huge, yellowish man made of rock.

Hal Jordan/Spectre: A giant Green Lantern battery.

Barry Allen: Unremembered, his sole purpose was to find a cure for the New God, Orion.