Answer: fiber or muscular fiber
Explanation: The things that impact the "tenderness" of the meat are things like the amount of liquid in it and the amount of muscle (formed by fibers). So if you want to make a cut more tender, you need to break down the muscular fibers on the meat.
An example of this is when a cut of meat is hammered with a meat mallet (or meat tenderizer) with the objective of breaking the fibers and make the cut more tender