We normally associate nodes with content and the relationships between them as edges.
It is a representation and as such, it emphasizes an aspect of reality.
Well, when thinking about another layer of abstraction such as dealing with the relationships between the relationships (meta-relationships) between the contents, which in the previous representation would be the edges, now, thouse, the edges, become nodes and the relationships among them are represented as new edges, drawing an independent graph but linked to the other and representative of another dimension of the same aspect of reality.