this doesn’t give a very good explanation but I’m sure there’s some good YouTube video that breaks it down. essentially maps out everywhere in space and time that could possibly interact with you in any way. this maximum is represented by how fast light can move away from you.
for example if you stole my car and ran away from me, I can draw a circle on the map every hour for how far you could have gone (assuming I knew my car’s maximum speed). if I put those circles on top of each other it’ll make a cone.