The equal a priori probability hypothesis (also called a postulate) states that an isolated system can be in any microstate that satisfies the macroscopic conditions, all with equal Probability. In other words, there is no "privileged" microscopic state. In a single sentence:
In a more geometric sense, this hypothesis is saying that the Probability distribution of states (equivalently, the density of representative points) over a constant-energy hypersurface in phase space is uniform.
This is a working hypothesis and not something that is strictly proven in classical mechanics, as even if it were proven from the equations of motion, we'd have to prove it again in a quantum field later to fully justify it. It is a natural consequence of the ergodic hypothesis.