In a nutshell, the idea is that you look below the surface of any problem/event, to figure out the underlying patterns, structures and mental models that led to the event.
You see it referenced a lot in relation to systems thinking, but the results that come up in a search feel a little bit sketchy. Where does it come from originally? It's mentioned obliquely in Thinking in Systems, so perhaps that's the genesis.
Like the tip of an iceberg rising above the water, events are the most visible aspect of a larger complex—but not always the most important.