Manhood
Kipling talks about manhood constantly, like all over his poetry—sometimes explicitly, sometimes not so much. While we have no idea for the first 30 lines of "If" that it's going to be a poem about being a man, it is very clear at the end that that is what it is. In many ways, the poem is a recipe for how to be a man and conquer the world: "Yours is the Earth and everything that's in it, / And---which is more---you'll be a Man, my son!" (31-32).
"If' isn't the only Kipling poem that talks about manhood, though. "The Thousandth Man", for example is all about brotherhood and friendship. "The White Man's Burden," a poem that has gotten Kipling in a lot of hot water over the years, is about leaving behind childish things and, well, becoming a (white) man: "Take up the White Man's burden-- / Have done with childish days" (49-50). And of course, we also see that same idea in that most famous of Kipling books, The Jungle Book, if in a slightly different way.