In this poem, the narrator is giving his son advice about living a good and worthwhile life and the manly virtues it takes to do this. He ends with:
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!
What he means by the "unforgiving minute" is time or life itself. If you can put the same unrelenting energy into it that you would put into a sixty second run, you can have "the earth." In other words, he is saying it is important to live your life fully and not hold back.
Throughout the poem, the speaker is offering advice about how to live well as a British gentlemen. Other attributes include not caring too much about money, trusting oneself when others don't, behaving with dignity to both kings and commoners, and shaking off defeats as if they didn't matter.