In short, a gorgeous piece of television. In serves as not exactly filer, but more as a whole platter of foreshadowing and symbols.
It begins with Willow, showing her safely with Tara, on the stage in a twisted version of a play featuring Riley and Harmony, and her insecurities concerning her witchcraft, lesbianism, and her former geekitude. She is accosted by a mysterious woman and her spirit sucked away.
Next comes Xander, who is shown in the typical male fantasies at first, with Joyce and also a scene with Willow and Tara, and then in a good adaption of a scene from Apocalypse Now featuring the early eaten Principal Snyder. He then visits the park, his ice cream truck, and Sunnydale High. Interspersed with these scenes, he keeps ending back up his parent's basement. There's also a very cool scene in which he moves through the various sets seamlessly. In the end, his heart is ripped out by the warrior woman.
Giles' dreams follow, showing a merge with the life he could have had with his former flame Olivia and his fatherly affection for Buffy. There's a brilliant scene with Spike acting as a sideshow attraction in black and white. He ends up in the Bronze and sings an expositional song before he is scalped.
Buffy ends the four, confronting Riley and the actor who played Adam, visiting her mother trapped in the wall, and venturing to the desert to converse with via Tara the woman who has been following her friends. She is and was the first slayer. They fight and the dreams end.
The whole episode is rife with subtlety but rewatching it with knowledge of the coming seasons makes it infinitely interesting. And of course, who can't forget the cheese man.