Castiel kept phrasing it as 'redeeming mankind' or 'absolving humanity', but what that actually means... I couldn't tell you.

Only time I ever heard Dean talk about it directly, he was drunker than I'd ever seen him, and said something about 'saving everybody'. Now, I've known a hunter with an ego or two in my day, but saving everybody ... that's a pretty tall order. Of the impossible variety.