A spiritual realm or afterlife, such as Heaven or Hell, are the place where good or bad souls go after death.
HeavenWhen Jonathan died, his spirit went to Heaven. Those who are in Heaven can watch over their loved ones. Heaven's gate is so strong that even Kryptonians have troubles opening it. Jonathan's spirit can leave Heaven whenever he wants.
There may be various existing Hells. Apokolips is viewed as a hell and the olympians god Hades rules over an olympian hell.
The Abyss of Life and Death
The Abyss of Life and Death is a spiritual plane of existence where wavering souls are transported.
When Clark is stabbed with the limbo drug, he enters Heaven. Clark has troubles opening Heaven's gate, but eventually manages to open it. He has a warm, heartfelt reconvening with his father, before he's pulled back to Earth.
Shortly after Clark Kent stabbed himself with a dagger with blue kryptonite, his soul arrived at the Abyss and was faltering on the edge of life and death itself. Because Clark didn't cross over right away, Jor-El was able to talk to him. After enduring Jor-El's condescending remarks, Clark saw Lex Luthor. Immediately afterwards, Lois Lane removed the dagger from Clark's body, causing the spiritual backlash to return Clark's soul to his body while Lex's soul went into his composite clone.