"If" is presented as a lecture of guidance and advice, delivered by a father to his son. The father is giving his son suggestions and examples of ways in which the son should learn to conduct himself in his dealings with others as he goes through life. The advice is presented as a series of scenarios, first presenting a negative reaction to a situation and then giving the contrasting positive behavior.
The father begins by suggesting personal qualities that are desirable, while warning against looking down on others who may not have those qualities. The goal is to achieve high personal standards without becoming conceited or condescending. The son is encouraged to be ready to develop worthy ideas and plans and take action to make them reality, while not allowing the criticism or ridicule of others to discourage him.
If you can bear to hear the truth you've spoken
Twisted by knaves to make a trap for fools,
Or watch the things you gave your life to, broken,
And stoop and build'em up with worn-out tools;
The son is challenged to remain true to his convictions, continuing to strive for the achievement of worthwhile ends even when others abandon the cause.
If the son succeeds in meeting these expectations, the father closes, he will achieve the ultimate rewards: "Yours is the Earth and everything that's in it, And---which is more---you'll be a Man, my son!"
The poem is written as four stanzas of eight lines per stanza, utilizing an "abab cdcd" rhyming pattern and iambic pentameter stress pattern with an extra unstressed syllable at the end of each initial "a" and "c" line. The language is very straightforward and common, as would be expected when a father is giving advice to his young son.