A much better question would be what he awakened from.

What do you awaken to in the morning? Hard to say, maybe we can call it “reality”? What do you awaken from? Ah, this is much easier - you awaken from your dreams, your fantasies. They may have been entertaining but none of that was real.

Same here. Buddha awakened from the dream of his ego (which persists during daytime, as opposite to night dreams) to the absolute reality.

Describing the absolute reality is pretty much impossible, and pointless. If, in your night dream you talk to a character who tries to convince you that there’s a reality on top of yours and tries to describe it to you, it might be entertaining but you won’t wake up. The identification with the dream must be broken, and it usually happens only in the morning (barring unusual cases such as lucid dreaming).

It’s not that you don’t know, on some level, that you’re dreaming. You know. Otherwise, phenomena such as lucid dreaming would be impossible. Remembering it is the hardest part, because we get lost in it.