Rudyard Kipling's poem titled "If--" is a list of conditionals expressing ways to behave in order to be the best possible human being, a real "Man, my son!" For example, using the collective "you," the poem talks about holding your head up high while also being humble, trusting in yourself while also allowing room for doubt, allowing yourself to dream while not letting your life be guided just by dreams, treating others equally, hating lies, and accepting the follies of others, among many other pieces of advice.
In the third stanza, Kipling uses the first line to speak of winning and taking chances, knowing that all men will be tempted at some point to make a foolish decision: "If you can make one heap of all your winnings / and risk it on one turn of pitch-and-toss ..." The game "pitch-and-toss" is a game of chance, so we know he's speaking of betting all your winnings on a gamble. However, what's important in this stanza is his advice for how a person should behave once you make the mistake of gambling all of your winnings and losing. You should "start again at your beginnings," meaning make a fresh start. You should also "never breathe a word about your loss," meaning never speak of the loss because not complaining about adversity shows strength of character and true humility.
He uses the next four lines of the stanza to speak of your strength of will. He advises you to keep making your body--"your heart and nerve and sinew"--do things long after they can no longer move. He also advises you to "hold on when there is nothing in you" that can keep holding on except for your will to hold on.
If a person can do both of these things listed in this third stanza, then that person proves to be a very strong, dedicated, and even humble person.