The study of how many smaller systems(components) interacting with each other and their environment can create a bigger system without the need for an centralized authority or external interventions(System Self Organization). The super system that is built might not be predictable by knowing all the smaller systems. You'll have to see the parts, the connections - and also how these connections result in a new system. The creation of new behaviors and properties like this is called emergence.
Simple System's function can be predicted by understanding all its components. Complex Systems, not so much.