Being a Jedi Master vs. a Jedi Knight seems to be primarily a point of experience & versatility. The moment that marks a Jedi Knight as ready for masterhood varies with each individual, but seems largely to depend on an understanding of the Force. Knighthood is concerned largely with the practical aspects of the Force, whereas Masterhood is more concerned with the philosophical aspects of its use.
Probably the earliest mention of this difference is Ulic Qel-Droma's death in "Tales of the Jedi: Redemption" some 4000 years before the movies...
Later references would be something like the scene from "Legacy of the Force: Betrayal" where Mara Jade has to do multiple tasks at once:
Yes, any Jedi Knight might know one of these three techniques - most
commonly, the technique of levitation. But only a Jedi Master was
likely to know all three or be able to sustain them simultaneously
across such a broad distance.
... or the scene in the same novel where Luke asks Jacen Solo to undertake a seemingly suicidal mission and explains why he chose him instead of a master:
Third, your specialized training in alternative philosophies of the
Force makes you more versatile than many other Jedi - than some Jedi
Masters, in fact - making it harder to stop you.