There are so many great comments here; I don't think it's possible to describe to someone who hasn't been there. It's not an intelligence barrier; the simple experience of parenthood is impossible to convey. That said, I'll give it a shot:
It's wonderful and terrifying and deeply satisfying and the most frustrating thing you'll ever do. Days and weeks will whip past with appalling speed; things that once seemed life-shatteringly important now appear quaint, humorous, and mildly embarrassing. You'll become a better person than you ever thought possible, and will become acutely aware of your every character flaw.
You'll never know if you're going to regret it, but you won't care.
It's wonderful and terrifying and deeply satisfying and the most frustrating thing you'll ever do. Days and weeks will whip past with appalling speed; things that once seemed life-shatteringly important now appear quaint, humorous, and mildly embarrassing. You'll become a better person than you ever thought possible, and will become acutely aware of your every character flaw.
You'll never know if you're going to regret it, but you won't care.