The speaker advises his son to be a man with strong belief in himself. If one is in the right, he or she must not be bothered by the allegations brought against them by others. Truth can’t lie hidden for too long. So, when he asks his son
“not (to) be tired by waiting...”
he is advising him to be patient. Also, it’s not necessary that you attain success on the very first attempt in everything you try. You need to be patient and should strive again and again to attain your goal. Patience with hard work is still the key to success.
Besides, the poet knows very well that it is a human frailty to lose self-control at times of success and failure. It demands a lot of mettle to master one’s dreams and thoughts. He is telling him the value of self-control when he says,
“If you can dream—and not make dreams your master;
If you can think—and not make thoughts your aim...”
The value of self-control can’t be denied. It is needed in every walk of life. One who exercises self-discipline is always appreciated. But if one allows oneself to be enslaved by the fickle mind and ever-changing desires, one courts disaster.
Moreover, the poet wants his son to understand that the two states of “Triumph and Disaster” are essentially transient in nature. Therefore neither of these two should be given much significance; instead they should be treated alike. He says to his son,
“If you can meet with Triumph and Disaster
And treat those two impostors just the same...”
While success may make one snobbish, condescending, arrogant and insensitive, failures may cause one to take to unethical and illegal methods.
That’s why the poet calls success and failure “impostors” and asks his son to treat them alike. If one is able to do this, they would be able to acquire a state of equanimity. Such persons are neither easily troubled nor can they be easily misled. Such people know the trick is to smile and remain satisfied in every situation in life, good or bad. However, not all find this state of equanimity; whoever has it, they experience an everlasting sense of satisfaction and life. This is still very much relevant.
Then, the poet talks about the value of fortitude. Everybody knows adversities and difficulties are part of life. If you have courage to deal with them, they may not cause you much harm. Thus, the value of fortitude that the poet teaches in the following lines is still very important in life:
“If you can make one heap of all your winnings
And risk it on one turn of pitch-and-toss,
And lose, and start again at your beginnings
And never breathe a word about your loss;”
“…so hold on when there is nothing in you
Except the Will which says to them: ‘Hold on!'”
In the final stanza, the poet talks about the importance of making optimum utilization of time. Time is the most precious of all one’s assets. So one must always strive to derive the most of every minute one's life. Nobody in the world can deny the immense worth that every single moment of human life holds.
Thus, in my opinion, you must not be doubtful about any of the virtues that the poet mentions in this poem. Each of these virtues is ageless and each of them has their special significance in human life. They are very much relevant today.