The quote is by a first-century Jewish scholar named Hillel, and it's equally applicable now as then. I love it, because it sums up how I have tried to look at my life, and it has served me very well. Let's break it down, shall we?
"If not you, then who?" Let's face it, someone has to step up. We're often faced with incompetent peers, unmotivated, unknowledgeable leaders, and subordinates who are a result of that environment (they are incompetent and unmotivated, primarily through lack of training and emphasis). And we also meet the "seagull leaders." You know, the ones who fly in, make a lot of noise, crap all over everything, and fly out again with no contribution (other than a pile of crap). Someone has to fill that leadership void. Let that someone be you. Like Patton said, "A good plan, violently executed now, is better than a perfect plan next week." A good leader, willing to take charge and make progress now, is better than a great leader further on down the road. And who knows? You might just be that great leader.
I've often found that, even if you're faced with a situation you aren't all that familiar with, someone in your organization knows the process and the techniques, and is just waiting for a leader to get them going. To provide them with the three hallmarks of good leadership: Purpose, Direction, and Motivation.
So step up. Get in there and take charge. Yeah, it's probably more work for you, and if you're like me, then you're a lazy sack. But waiting won't help, and will probably make things worse. Which leads me to my next point....
"If not now, then when?" Problems, like fish, do not smell better with age. Waiting will not make it go away, and will not improve your lot. So if you're going to be that guy (or girl), then you need to be that guy right now or risk having things spiral completely out of control and be unrecoverable.
The cool thing is that this part of the quotation is completely dependent upon the first half. You have to be willing to step up, to assume the difficult role of the leader, and to take responsibility for successes and failures. And you have to do it NOW, on the spot, because waiting for conditions to improve isn't going to help anything. Systems tend towards entropy, whether we're talking about collections of atoms or people. And unless you are willing to impose order on the chaos, right now, you're not a useful leader.
Men's Health magazine did an article on heroes. I don't have time to find the link right now, but you should look it up, because it's really good. And the bottom line for these guys (the heroes) is that they're "ordinary" people in their own minds, who saw a problem and figured something had to be done now, so why not just do it?
OK, so on to practial applications. For me, (duh) it comes down to the Army again. They tell us as privates not to volunteer for anything. To be middle-of-the-road, and to avoid notice. If we do that, we hear, we'll be successful. Hooey, I say. If everyone's middle-of-the-road, then who's in charge? Who do we follow? I have volunteered for a lot of stuff in my Army career (I'm sure Jen can give you a rundown, and then she'll kill me for reminding her), and every time, I come out ahead. Yeah, it means more work. Yeah, it's long hours and not a lot of reward (other than more work). But the mission gets accomplished and the Soldiers get taken care of, and I end up a better Soldier, more knowledgeable, and more able to handle situations in the future. In short, I become a better leader.
It works in non-Army situations, too. Who is going to mow my lawn? Is someone going to come around and do it for me? What about paying my bills? Unless I win the McDonald's Monopoly game, no one is going to hand me money.
What about helping out the lady in the ward who can't switch over her swamp cooler by herself? Or landscaping the yard of the woman whose husband is deployed and needs it done so she can take in some foster kids?
We, the leaders, have to step up. We have to step up now, and get these things done. Because no one else will, and no one can do it as well as we can.