So you have some people sitting around and talking. Some of the things they say are about fictional characters in a fictional world. During the conversation the characters and their world aren't static: the people don't simply describe them in increasing detail, they (also) have them do things and interact. They create situations - dynamic arrangements of characters and setting elements - and resolve them into new situations.
They may or may not have formal procedures for this part of the conversation, but the simple fact that it consistently happens reveals some sort of structure. If they didn't have an effective way to negotiate the evolution of situation to situation, their conversation would stall or crash.
Why are they doing this? What do they get out of it? For now, let's limit ourselves to three possibilities: they want to Say Something (in a lit 101 sense), they want to Prove Themselves, or they want to Be There. What they want to say, in what way they want to prove themselves, or where precisely they want to be varies to the particular person in the particular moment. Are there other possibilities? Maybe. Certainly these three cover an enormous variety, especially as their nuanced particulars combine in an actual group of people in actual play.
Over time, that is, over many many in-game situations, play will either fulfill the players' creative agendas or fail to fulfill them. Do they have that discussion? Do they prove themselves or let themselves down? Are they "there"? As in pretty much any kind of emergent pattern thingy, whether the game fulfills the players' creative agendas depends on but isn't predictable from the specific structure they've got for negotiating situations. No individual situation's evolution or resolution can reveal a) what the players' creative agendas are or b) whether they're being fulfilled. Especially, limiting your observation to the in-game contents of individual situations will certainly blind you to what the players are actually getting out of the game.
That's GNS in a page.
I don't think I've said anything here that Ron Edwards hasn't been saying. I do think that I've said it in mostly my own words.