I believe it refers to the way the cities expand naturally. You get a new tile after you accumulate enough culture points in that city. You can't choose what tile gets 'bought' next that way though (it will be one of those purple hexes in the city view). The use of those '-marks around the word "buy" could refer to that as well...
Thanks for the responses. Yeah, I was aware of the expansion from a city's culture, but the CFC's "Civ5 Confirmed Features and Versions" says:
"Borders now only expand at one hex at a time and more difficult terrain will take more time to claim (Swedish PC Gamer). You can use Gold to sped up this process, but that may bring you into conflict with surrounding Civs. GamerNode
You can also use culture points to 'buy' tiles
Gamespy"
This seems to present "expansion" and "buying tiles" as two separate ideas. What confused me more, though, is the link from gamespy. If you click on it, you go to an interview with a gamespy editor who had just seen a detailed presentation about Civ V. He says:
"I also thought the fact that you can buy tiles with culture points, and direct cultural expansion exactly where you want, adds another drill-down layer..."
If he means expanding your borders through your city's culture, that's a strange way of putting it. I guess "direct cultural expansion" refers to buying the tiles with gold? I dunno, just seems like a convoluted explanation. There's no "buying" of tiles with culture points, only cultural expansion. I guess that's what he was trying to say; although I wonder if maybe he was just confused about how it actually worked. I mean, "buying tiles with culture points" sounds like a pretty specific idea...kind of far from your culture points causing border expansion.
Oh well, guess I've harped on this enough. Thanks.