The Source Wall is the edge of the universe.
The Source Wall was a massive construct deep in space, with large statues within it. When Darkseid launched an attack on Earth to avenge himself against Superman, Lex Luthor compelled Metron to help him defeat Darkseid. Metron took him to the Source Wall, beyond which lay the one thing which could defeat Darkseid. Metron warned Luthor that only a twelfth level intellect could survive passing through the wall, but Luthor was unconcerned. Luthor did manage to survive his trip through, and found the one thing that could stop Darkseid—the Anti-Life Equation.