I guess Ready is kind of an indicator to prepare yourself and Set would be an indicator to position yourself. One is mental and the other is physical.

It is short for "On your mark... get set... go." I am not an expert, but these are the commands used to start a foot race, like the 100-meter dash, in track-and-field competitions. "On your mark" means to put your feet against the starting blocks. "Get set" means to crouch your body into a starting position. "Go" means to leave the blocks and start running.

This is usually used in things like running races where the competitors start from a special stance. So if you watch a race you will see that on “ready” the runners will bend down into a crouch, on “set” they will tense their bodies ready to run, and then on “go” they take off.