Answer: It is the speed at which a push will propagate to the other end. So if you push a long stick, the far end will not start moving until some milliseconds later. But if you put a constant acceleration at one end, the other end will accelerate constantly as well. If you push long enough for one end to break the sound barrier, the other end will break the sound barrier a few milliseconds later. While being pushed, the stick will be slightly shorter, and it will regain that length (if it has not burned up) when the pushing stops.
Step-by-step explanation: