It’s what you can infer from HOW characters say things. What they think of themselves, the other characters and the plot can be conveyed with their strong personal voices.
It’s what you can infer from what the characters DON’T say. A main character should confess accurate self-knowledge only after the emotional catharsis that changes them. At the END.
It’s what you can infer about pacing, tension and stakes without a recap.
It’s what you can infer about the specific dramatic conflict in the scene.
Subtext is the polished, thoughtful, professional big sister of flat exposition and you should be using a lot of it in your script.