I think you are talking about the line in the last stanza:
If you can talk with crowds and keep your virtue,
Or walk with kings - nor lose the common touch;
If neither foes nor loving friends can hurt you;
If all men count with you, but none too much;
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!
Unforgiving minute suggests a minute in time you will never get back again. It does not "forgive" - meaning, if you mess up, you don't necessarily get a "redo." So here, the speaker advises to fill it with the same endurance and fortitude (and with the same attitude) you've devoted to the rest of the "distance run" - or any other minute. In a way, this is a final word saying, "Don't give up at the last minute. Finish strong!"
I assume that you are asking about the meaning of the phrase "unforgiving minute" in the last stanza of the poem. In my opinion, what the author means by this phrase is that we never get another chance at any minute in our lives. Once a minute is gone, it will never come back and we will never again have an opportunity to do something different with that minute that just passed.
So, in that way, the minute is unforgiving. You get one change to do the right thing in that minute. Once it is gone, you cannot change what you have done.
It means that time is unforgiving in the sense that whatever we do, time will pass and take us closer to our death (the pessimist's view on life). It is a message to the reader that we should all live every minute like it is our last because we never know when our time will end.