In dialog and narrative both there is always both text and subtext. The first is what is said outright, and the second is what is implied by what is left unsaid, or what qualifies what is said. Often, subtext, which the reader picks up on, is as important as what is actually said. Look at the following:
“Oh, is that slide show at the library with the nature photographer tonight?” John grimaced. “I’ll go if you want, but I’m really tired. After all, I was out to the Bible Study at church last night and you stayed home and read. This makes two nights in a row, for me. Of course, I don’t like to mess up your plans....”
Okay, here the subtext is pretty plain: John is hostile and disagreeable and of course he wants to mess up her plans. If he didn’t, he’d say, “You go ahead, hon, I’m too tired tonight." What this really says is, “You couldn’t find time to go with me last night, so I’m NOT going to be nice about what you want to do tonight.”
That is the subtext, although the text says that he'll go if she wants, it is made clear by the subtext that he doesn't really WANT to go and doesn't want her to go and enjoy herself, either.