All teeth are white, and everyone is the same when it comes to teeth—we all have 'em. Meanwhile, White Teeth is populated by characters from different races and ethnicities and of different ages, who speak different languages and have very different ways at looking at the world.
So we think teethwork is a symbol of humanity in this novel. You've probably heard a different version of this same symbol many times before: everybody's blood is red. So we all bleed the same blood. Same idea here.
Yet, at the same time as teeth are a symbol of humanity as a whole, teeth are also used to identify individuals (yes, like on CSI). Each person has a unique set of teeth. And teeth also have roots. And roots, as all the talk of root canals suggests, are worth saving.
So, Zadie Smith is demanding kind of a lot of us readers with this teeth symbol. She wants us to just sort of hang out with the idea that people are basically the same (you know, there's only one humanity), but that there are also some really awesome differences between people.