"If" is filled with advice on how to best spend your time, and best react in each situation that is presented to you, no matter how diverse it is. So, when Kipling states, "If you can fill the unforgiving minute/With sixty seconds' worth of distance run," he is saying that with every minute that you are given, make the absolute most of it that you can. "Unforgiving minute" refers to the fact that every single minute is 60 seconds long-no more, and no less. So when that minute is up, it is gone, forever. You can't call it back to spend that time differently. A minute is not merciful; it doesn't slow itself down to give you more time, or tack on a few seconds, or take a few of here or there. It is unforgiving time; always constant, always running. So, Kipling's advice is to fill every minute "with sixty seconds' worth of distance run," or to get as much good, effort, energy and distance out of every minute that you are given. I hope that helps!