Winston's answer got me pondering this, and I'm still pondering, so the answer may evolve. But instinct is telling me:
- It means painful sacrifice, in addition to hard work and physical pain.
My teacher used to say "Whole life. Full time. Only this." This meant giving up most of what the rest of us enjoy, such as active social lives, hobbies, time off, etc.
There's a saying "Either the family suffers, or the career suffers", and I suspect there is a parallel.
For the overwhelming majority of people, martial arts is not a good way to make money, such that even the decision to teach full time for many/most is not an optimal financial decision. It's a hard life, and, for most, there will be very little recognition.
This is what Olympic athletes do—sacrifice the other parts of their lives to focus solely on that one thing, for as long as they can do it. And of the potentially hundreds of athletes in an Olympic competition, only a few will win medals, and fewer still the golds. Yet every single one of them is an extraordinary athlete, the best of the best.
I don't think this means the rest of us have to give up everything, because most will not be Wayne Gretzky no matter how hard we work.
- I've always heard the phrase used to connote "good" skill that anyone who is dedicated enough should be able to attain, not exclusively the "greatest" skill.
It could could mean that we should try to focus on it to the exclusion of every possible other thing for at least some period of our lives, to have that experience, training and extra edge.
- Part of it might be realizing we'll never quite get to where we want to, and maybe most of us aren't quite as good as we thought, but we keep on going, regardless.
After all of that, maybe find we have something of value—"good gong fu."