Unlike Saussure, who just looked at structure as linear, Derrida insists that all structures have some sort of center. He's talking mostly about philosophical systems or structures, but the idea applies to almost any structure. There's something that all the elements in the structure refer to, connect to, something that makes the structure hold its shape, keeps all the parts together. (You should note, though, that this model DOESN'T work so well for language-- or not for Saussure's idea of language--where it's difficult to locate or name what "center" might hold the whole structure together).