It means knowing exactly where you really are in life - without lying to yourself about what you are really like - and then setting your directions from there, rather than from some idealised version of yourself that never really existed.
It means that rather than making decisions on the basis of wishful thinking, you work out what the cost will be of getting the thing you want and make a conscious decision as to whether that's a cost you're willing to pay.
It means putting in the effort to understand world around you as it really is, not as we wish it was, and doing what's necessary to cope with that. And, sure, trying to change it for the better - but from a place of knowledge, not assumptions and bias.
(None of this is easy, and nobody gets this right all of the time - I sure as hell don't. Adulthood is an ideal to strive for, not something you achieve. Oh, and it shouldn't stop you from having a damn good time and engaging in child-like fun along the way.)
Original post on Dreamwidth - there are comments there.