Despite having become such a central part to the Dragon Ball Universe, the Super Saiyan transformation is in many ways just as mysterious and enigmatic as it was when Goku first reached it against Frieza in Dragon Ball Z. At that time, the excitement of Goku's god-like power was enough to push questions off to a further date, but as time went by, not much more has been definitively explained about it.
Source：https://www.cbr.com/dragon-ball-super-saiyan-nonsense/
