As someone else said, he said "all is revealed". Not sure if there is any special meaning but if there is--it's that Phoenix is an ancient cosmic entity that is one of the "forces" of the Universe. The "Celestials" are beings on this level, not as powerful as the Phoenix, but on the "cosmic entity" level, and there is an entire race of of them. To give a point of reference, Odin created a super sword from the energy of all the pantheons on earth, so like the Odin Force*10, and shot the head Celestial with it--and the Celestial laughed. (Though, to be fair, the head Celestial is ridiculously more powerful than the rest. A few of the Celestials are probably as powerful as Phoenix, especially Phoenix in Jean's body. But the "average" Celestial is significantly weaker. I think like 4-5 no name Celestials were able to fight off Galactus, but a couple died. However, the leader Celestials could probably easily take most versions of Galactus.)
Any case, the Celestials made Apocalypses armor in the comics. In the Comics, his armor is the source of most of his powers--it gives him SOME powers of a celestial. In effect, the person who wears that armor is supposed to be a herald of the Celestials. So Apocalypse has some understanding of what the Phoenix Force, Eternity, Infinity, Galactus and other Cosmic entities. (In the main Marvel films they call all of these Celestials, but in the Comics Celestials are a specific thing--the city nowhere the Guardians go to? That's the head of a Celestial.)
Here is one in the of them in Guardians.
That's
Eson the Searcher.
Any case, so Apoc knows of these beings somewhat. He knows there are creatures that are actual "Gods". If this meant anything at all it was his acknowledgment that he was going to suffer the same fate as the people he was fighting--IE that the Phoenix was as far above him as the he was above the mutants. Which is one of his best lines in the Cartoon (Not sure if he said it in the Comics.)
But you can see from the Cartoons how badly they fucked Apocalypse up. I doubt the line actually meant anything in particular except his way of say "I fucked up". But if there was some larger reference it was the fact that he had a good idea of what was about to kill him and how badly he'd fucked up challenging it.