The idea that events
in the future may or may not affect
you. If an event is not in your event cone, it will not affect you.
I| X.X b
The "shaded" region inside the X's is your event cone. Events within this area can possibly affect you. Anything outside that cone cannot.
Example: A can possibly affect you. It has the time needed to cover the space to affect you. C has already affected you and is in your past. B cannot affect you, since it does not have the time to cover the distance needed.
I learned this all in physics
the other day and thought it was pretty darn neato