A light cone is the path that a flash of light, emanating from a single event localized to a single point in space and a single moment in time and traveling in all directions, would take through spacetime. Because signals and other causal influences cannot travel faster than light, the light cone defines the concept of causality: for a given event E, the set of events that lie on or inside the light cone of E would be the set of events that could receive a signal sent out from the position and time of E, so the light cone contains all the events that could potentially be causally influenced by E. Events which don't lie in the light cone of E cannot be influenced by E, they are in the "elsewhere" region. If we are at E, we cannot causally affect or be causally affected by an occurrence at an event in this "elsewhere" region.
Now imagine that you have born at point O at a single moment in time. All your possible futures will be inside the light cone determined by the path of light traveling in all directions with origin in O. Whatever you do, you will never be able to break through the light cone because you can't travel faster than light. All your projects, dreams, realizations, will be inside the light cone: it is your existential prison.