The Expanse

It is a bit cryptic that phrasing. I think they are trying to describe a null/light geodesic, that is the path light follows in space time, in a geometric way. Or maybe they are comparing to light to a vibration that has to travel through a membrane? I do believe they make it clearer later what they mean in the context of superluminal communication/phenomena. The point is something like this: if you take a stick and have a point a and b in the sand, draw a line between a and b as fast as you can, it will always take some time larger than zero. This speed limit is the speed of light. Now if you instead drop the stick so it goes from a to b, you have managed to mark the path a to b faster than drawing it. Sending light, you drawing, will be bound by some speed, their communication, the equivalent of just dropping the stick, will not.