A light cone is the cone extending in timespace in front and behind of any event. The boundary of the light cone is the boundary of events that could have possibly influenced the event in question. For example, an event which is a light year away from a test location couldn't influence events at that location for a year. (Ya, I know it's more complex than that -- see a Relativity text for the full story.) The forward light cone is all events that the given event can influence.