It does not say anything about the actual moment of resurrection. What was it actually like when Jesus arose. Hoe exactly did God accomplish this? What did Jesus feel, see, experience as life returned to his body? It tells us that an empty tomb was discovered and that after that the disciples encountered Jesus alive.
I assume that it means that this was the moment at which he believed in what Jesus had taught him about the Son of Man being raised from the dead. What we see in the Gospels is that the disciples became very upset with Jesus when he spoke about his pending death, suggesting that they weren't even willing to think about resurrection. This was a sort of "Oh! Now I get it!" moment.
John 20:9 is an interesting verse indeed. Most translators and commentators that I'm familiar with take that verse in the sense of not understanding the Scripture rather than not knowing it, but it still begs the question: what Scripture? By the time John wrote Christian Scripture was still largely undefined and in flux, with different Christian communities having different "canons." I've discussed the development of the Christian canon before, so don't propose to go into it again here, except to say that "Scripture" today (in mainline Protestant churches at least) is the 66 books of the Bible from Genesis to Revelation. There's lots of Scripture there that talks about the resurrection - but none that "describe" it in the sense I'm meaning that I'm aware of. If by "Scripture" you mean extrabiblical writings, I'm still not aware of an account of the actual moment of resurrection.
To me, it seems perhaps more likely that the point in John 20:9 was that the disciples hadn't understand from prophecy (ie, the Old Testament) or perhaps from the words of Jesus (by the time John was written, it's likely that the synoptic Gospels at least had been accorded sort of "quasi-canonical" status) that this was going to happen. Not entirely clear, though, what "Scripture" he is referring to.