Collections of nodes in the graph can be collapsed into sub-graphs for organizational purposes. This creates a hierarchy of graphs and allows large or complex portions of a graph to be viewed as a single node in the parent graph, with inputs and outputs, while still being able to edit the contents of the collapsed graph.
Unlike macros, a set of collapsed nodes is not shared, even within a single Level Blueprint or Blueprint Class. If you copy the collapsed node, it duplicates the internal graph. This can be handy if you want to make several variants of the same approximate behavior, but it also means that any bug fixes would have to be applied to each copy. The feature is really more intended to 'tidy up' a graph, hiding complexity inside, rather than any sort of sharing or reuse.