The Shadow Realm is an other dimension parallel to the one Earth inhabits and it first appeared in the episode "Kids Stuff" of Justice League: Unlimited. It is an endless void with floating pieces of rock.
When Mordred took the Amulet of First Magic for himself and gained tremendous powers, he banished all of Earth's adults including his own mother Morgaine to the Shadow Realm.
Amusingly, Copperhead mistook it as Hell and thought that Judgment Day arrived. Superman, Wonder Woman, Batman, and Green Lantern were approached by Morgaine Le Fey to defeat Mordred. To do so, she cast a spell to turn them into children and bypass her son's spell.
After Mordred when he turned himself into a teenage adult, was teleported to the Shadow Realm, the spell was broken and everyone returned from the Shadow Realm to Earth.