ESC is an acronym for Electronic Speed Controller and it serves a purpose of obviously controlling the speed of which each motor turns. Since the motors are tri-phase induction type motors, they cannot be controlled by just applying power to them. Each motor needs its own computer to tell it how many degrees to turn in steps. If this command happens very quickly, you get a full, smooth rotation.
Until recently every motor would get its own module, meaning for 4 motors we'd need 4 individual computers but with recent advancements in drone racing technology we can now get all four into a single board. Since this particular drone will be quite powerful, we also need a decent speed controller. The best candidate for the prototype is the HakRC 60a 32 bit board. It can handle 70 amperes in short bursts and it boasts a 32 bit computer on board.
It is by no means the best one on the market and the final product will need a better, more reliable unit such as the T-motor's ESC lineup which can often be double the price, but for now it will do.