The cosmic event horizon is the set of points from which light is just reaching us now. Anything further away cannot be seen, as the light hasn't reached us yet, and anything closer can be seen. The horizon is expanding at light speed, as something being behind the horizon by, say, one light second, means its light is one second from reaching the observer, so one second later, it would be on the horizon. As nothing can travel faster than light, the cosmic event horizon can never be reached.