Light cone is a concept used in physics (including relativity), indicating the portion of space-time from which light can reach you (the further back in time, the further that distance is) or that light from your position can reach another position (the further into the future, the further that is). The light cone for a point in space-time excludes every other region of space at that instant (light hasn't had enough time to reach you)
More than light or electromagnetic radiation is involved: general relativity posits that no influence of any kind moves faster than the speed of light in a vacuum (c), thus portions of space-time are unknown to you. So if the Sun emits a solar flare right at this instant, there is no way to know that until the few minutes pass until light reaches Earth.
With the concept of relativity, with space and time not entirely independent, and modern cosmology assuming space's growth from a point, there are complications and consequences. A deduction that is made in astrophysics is that a transient that appears to result from release of a certain amount of energy over a short time (based upon the intensity of the light reaching us) implies energy released from a sufficiently small size to be triggered, to cause such a simultaneous release of energy. This is used as evidence for an object sufficient to provide the necessary power, e.g., black hole of sufficient size. Another consequence is the need to explain any observation that suggests a phenomenon in the early universe too widespread to be triggered at or after the Big Bang.