the best example is our complex brain being made of simpler components working together. maybe the opposite of complex is chaotic? i don't know...
The analogy is a lego castle vs a wool castle. A lego brick is very simple and contained, and from this you can build wonderful structures; in addition if you wish to change out a portion it is easy to do because changing on part of the system (i.e. implementation) doesn't affect the rest so long as the contract between components is maintained.
Contrasting: should you pull on a thread in a wool castle it will affect other parts of the castle. A lot of software is like this, which makes it very hard to reason about.
One technique is redefining the problem, to smaller or bigger:
Work on only part of a problem, a subset, leaving something out. e.g. git's data model does not represent renames as first class constucts, enabling it to be disproportionately simpler.
Expand the problem, a superset, to integrate lower or higher level or associated parts that aren't usually included. Previously scattered commonalities may then appear, enabling decomposition.
Cynefin would agree!