The meaning under the text. Here's an example from Woody Allen
's Annie Hall
. The subtext is directly under the text, in italic.
Oh, God, nobody cool says Hi.
I'd love to take her out. I hope my deodorant's working.
He's talking to me! And he'll hate my silly dress.
(Taking a furtive step toward her)
Just look at the clouds over there.
I've got to find out who she is.
I'm Leslie ...
What a stupid name. He'll hate it and hate me.
I just love her name.
The example is about a sexual attraction, but there are tons of other things that it could be about, anything which is not being said out loud about a topic that is present in the context.