Light cones are a concept used in physics (including relativity) for 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 it or vice versa).
Though called light cones, they set limits on more than EMR: 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 and the associated 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 (Big Bang), there are complications and consequences.
Astrophysics takes as fact that a transient showing a certain luminosity-increase must have a triggering event which could "turn on" the power-increase across an area facing us, but can only possibly affect any portion of this area when it is reachable at the speed of light, and a rapid increase in power sets a maximum area on the source of this energy, i.e., it must be some object able to produce sufficient energy, despite the object's limited size. Thus extreme transients have constituted evidence for black holes.
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, such as the uniformity of the CMB.