Causality imposes an ordering on events: cause comes before effect; a message is sent before that message is received; the question comes before the answer. And, like in real life, one thing leads to another: one node reads some data and then writes something as a result, another node reads the thing that was written and writes something else in turn, and so on. These chains of causally dependent operations define the causal order in the system—i.e., what happened before what.
