In If, how should he make allowances for what others doubt?
In Rudyard Kipling's poem If, the speaker makes certain allowances based upon his (assumptive given the author's gender) views of others set against that of the individual (his son). (The term allowances refers to the considerations one makes when attempting to modify his or her circumstances or the possibilities of what may come.) Therefore, the speaker considers many different things when detailing what one must do to "be a Man."
The specific allowances made are those which speak to the importance of remaining virtuous, rigid in one's morality, and willing to take chances. The speaker is basically saying that by keeping one's head, taking a chance on risk, having dreams (but not allowing them to rule life), and can "bear the truth" then one will be able to grow into a man and rule the Earth (not as in ruling people, but their own life).
These allowances are important because the author is offering what he sees as important in growing up, instilling values, and holding true to one's morality.