A light cone marks the boundary of everything a single event in spacetime could possibly affect or be affected by. It gets its name because if you plot every direction light could travel outward from that event over time, the result forms a cone-shaped surface.
Everything inside the future light cone is a place that event could still influence, since a signal from it could arrive there without exceeding the speed of light. Everything inside the past light cone could have influenced that event. Everything outside both cones is causally out of reach entirely.
Light cones replace the old idea of instantaneous, universal cause and effect. In relativity, influence can only spread at or below the speed of light, and the light cone is the geometric picture of exactly how far it can reach.
A useful way to picture this: when astronomers observe a supernova exploding in a galaxy 10 million light-years away, that explosion sits on the edge of our present-day past light cone, meaning its light is only now reaching Earth even though the star actually died 10 million years ago. Anything happening right now in that same distant galaxy sits entirely outside our light cone and will remain unknowable to us for another 10 million years at minimum. This is not a limitation of our telescopes, it is a hard structural feature of spacetime itself, and it is why physicists say the past and future are relative to a specific event rather than universal categories shared instantly across the whole cosmos.
Physicists sometimes discuss a hypothetical particle called a tachyon, which would always travel faster than light and therefore sit entirely outside any observer’s light cone. No tachyon has ever been detected, and most physicists suspect the concept is not physically realizable at all, but it remains a useful thought experiment for testing exactly how seriously relativity takes its own speed limit.