Buffy is meant to signify the character's flighty, shallow beginnings, when she's just a ditzy high schooler with little promise. The name Willow is a nod to that character's Wiccan practice and her deep spiritual connection to the natural world. A willow tree is one of the most sacred symbols for ancient civilizations, so it's not surprising she would be called this.
Xander's name has a much more concrete, albeit comic, history. Zander, a version of the name Xander, is a species of fish. This just so happens to be a direct throwback to the original 1992 movie of the same name. In the movie, Luke Perry played Pike, Buffy's stoner love interest, and it's no coincidence that a pike is also a species of fish.
There's also Angel, whose name reflects the irony of his constant spiritual conflict, as the only vampire with a soul.
