The stanza goes as follows:

If you can fill the unforgiving minute 
    With sixty seconds’ worth of distance run,
  Yours is the Earth and everything that’s in it,          And—which is more—you’ll be a Man, my son!

In the poem "If" by Rudyard Kipling, what does "unforgiving minute" suggest?

"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!

What does "unforgiving minutes" mean in Kipling's poem "If"?

One of Rudyard Kipling's most well-known poems, "If" is a homage to those who rise above their circumstances and make the extra effort to do the right thing, confident in their ability to prevail over adversity.  It's about going through life with the right attitude and not letting others drag you down to their level.  It is these sentiments that have made it a classic of Western literature.

If you can keep your head when all about you are losing theirs and blaming you;
If you can trust yourself when all men doubt you,
But make allowance for their doubting too...
Or being hated, don't give way to hating...

"If" is a poem that inspires individuals when they are confronted by doubt, or face a fork in the road and are tempted to take the path of least resistance.  

If you can fill the unforgiving minute
With sixty seconds worth of distance run,
Yours is the Earth and everything that's in it,
And -- which is more -- you'll be a Man, my son.

When Kipling uses the phrase "unforgiving minute," he is alluding, as throughout the poem, to the difficulties to which all men are subjected, and the value in trying one's hardest to prevail.