It isn't what your characters say; it's what they really mean. "Subtext"--what characters are saying between the lines. The subtext is all the underlying drives and meanings that are not apparent to the character, but that are apparent to the audience or reader.
Paraphrased from Subtext: The Delicate Art of Doublespeak
Subtext is a fascinating and intellectually stimulating tool in the prose arsenal. This raises the narrative from mere telling what happened, to asking the reader to think about what is being presented and look deeper into the meaning of the words and actions.
Here is a scene with subtext.
Man puts a log on the fire. His wife says that she wasn't cold, that there is no need to stoke the fire further. He responds that he puts a log on the fire every night, that the house will grow cold otherwise.The focus of the argument is about handling the fire in the fireplace, but the subtext is a deeper tension between the wife and her husband.
"Then put on a blanket," she shoots back, "or a sweater. Why do you need to fiddle with the fireplace."
"It's just a moment, the fire is already burning. Another log doesn't take any preparation."
"You've got to clean the fireplace. Don't you even think before you put on the log!"
"It's just a damn log. Fine I'll take it off, if that will make you happy."