"If" is a didactic poem. It teaches life lessons through illustrations. The speaker is teaching his son how to become a man. In the first stanza, the speaker encourages his son to have self control. He insists that the son keep his cool when others are losing theirs. He instructs his son to have patience. He encourages his son to avoid lying and hating. No doubt, this poem is filled with valuable instruction.
The second stanza encourages the son to dream but to not make dreams become master over you. He personifies Triumph and Disaster. The father instructs the son to treat both of these imposters as the same. He is is instructing the son to learn from both Triumph and Disaster.
In the third stanza, the speaker instructs the son to learn how to lose it all and begin again without complaining:
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:
The speaker continues to instruct the son to keep his nerve and to hold on when he feels like giving up:
If you can force your heart and nerve and sinew
To serve your turn long after they are gone,
And so hold on when there is nothing in you
Except the Will which says to them: "Hold on!"
If the son will learn from his fathers instruction, he will inherit the earth and become a man. That is what the poem is saying. The father is trying to teach his son to become a man.
In the last stanza, the father instructs his son to be confident enough to walk with kings yet remain humble. He encourages the son to protect his heart and not allow friends nor foes to hurt him. If the son will follow these simple instructions, he will become a man:
Yours is the Earth and everything that's in it,
And---which is more---you'll be a Man, my son!
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