The short answer is: a bad translation of a word that means “awake.” It’s not that the Buddha attained something we call enlightenment. It’s that he woke up. It’s like when you’re dreaming — you think you’re walking on water, or flying over Mars, but then you wake up and see where you are — oh, I’m in bed. There’s the closet. There’s the door. When you wake up, you see what’s actually there and not what you think is there.
Buddha’s waking up was like that, except what he woke up to was his original nature, which is your original nature, which is the nature of the universe — things as they actually are and not as our thoughts distort them.
You can wake up too. That’s the point of Zen practice, to help us cut through the distortions of our thinking and wake up, just as Buddha did.
And, just as every night you fall asleep and have to wake up again every morning, waking up is something we do over and over again. It’s not something we do once and then say: okay, been there, done that. It’s something we keep doing throughout our lives.