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.
T| X...X 
I|  X.X  b
M|   X
E|  X.X
 | X...X
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.