In the classic Twilight Zone episode "It's a Good Life", six-year-old Anthony Fremont (played by child star Billy Mumy) has the power to wish for anything. He often gets angry at other people, punishing them by sending them to "the cornfield". The other people in his town fear him, and do what they can to please him.
Was the cornfield an actual place that physically existed? Or was it a metaphor for oblivion?
The original short story describes the cornfield as a literal place where he sends the bodies (and this something that his family convinces him to do rather than leave the twisted remains of what was once human on the carpet), so I believe it was likely literal in the TV presentation as well.
From the point of view of the inhabitants, they may not know whether the "cornfield" is real. It's just where they tell Anthony to send things when they need to be gotten rid of.
From the opening narration:
On a given morning not too long ago, the rest of the world disappeared and Peaksville was left all alone. Its inhabitants were never sure whether the world was destroyed and only Peaksville left untouched or whether the village had somehow been taken away.
So the people living there don't know anything about what exists beyond the small confines of their town. It's not clear specifically by what means Anthony keeps them from exploring—whether it's merely out of fear that they don't try to get away, or whether he has placed some kind of barrier between the village and whatever else remains of existence. However, they seem to have no way of knowing where, if anywhere, the stuff sent to the "cornfield" actually ends up.
I always assumed that "The Cornfield" was, a real place, one of the many cornfields and other fields surrounding Peaksville that grew the food to keep the people there alive. I assume that Anthony transports his victims - living or dead - into the ground below "The Cornfield", thus helping to fertilize it.
Of course if Anthony transported the dead bodies of his victims onto the surface of "The Cornfield" the Peaksville people would avoid it and it would be abandoned and disused.
Coincidentally, all persons interested in the US Civil War recognize the phrase "the Cornfield" as referring to Mr. Miller's cornfield near Sharpsburg, Maryland, scene of some of the bloodiest fighting at the Battle of Antietam. This may have suggested "The Cornfield" as the phrase to described where Anthony sends his victims.
Sometimes Anthony explicitly kills things. The episode begins with Anthony explicitly telling the three-headed gopher he'd created and was playing with to "be dead".
ANTHONY: I'll make him dead now. I'm tired of playing with him. Be dead. Gopher, you be dead!
MAN: It's real fine that you done that.
Later, an angry man accuses Anthony "You murderer!" Anthony changes an angry man into a grotesque human jack-in-the box, his father kneels down next to Anthony and says (with perhaps seeming compassion?)
FATHER: Wish it into the cornfield. Please son, wish it into the cornfield. Please!
ANTHONY: He was a bad man.
We can't be sure if Anthony's father simply wanted the grotesque creation out of sight, or if he felt this would terminate whatever level of life or awareness the jack-in-the-box/human conglomeration maintained.
See also @FuzzyBoots' answer
There may be some further clues from the original story.
According to YouTube Channel Fever Dreamland Theater's It’s A Good Life - Twilight Zone Episode REVIEW the story first appeared in "STAR Science Fiction Stories Number 2" edited by Frank Pohl as Jerome Bixby's short story of the same name, so there may be clues.
I'll try to get a copy tomorrow, but if someone has one handy please feel free to edit here or write a new answer based on the short story.
In the original episode, Anthony states that he sent the dog, that was annoying him, "...to the corn field, he's not outside anymore." That's rather telling that the corn field is "away", and likely NOT part of Peaksville.
