"If": a simple, two-letter title, and one that does and does not tell us a whole lot about the poem. When we come across this simple little title, our immediate reaction is naturally, "If what?" This is because "if" is one of those words that really needs a friend, some other words to go with it. If you just walk into a room and go "if," people might look at you like you're from Mars (or Jupiter, or even Pluto). Right off the bat, then, the poem's title puzzles us. It is enigmatic, to say the least. (Note: this does not mean Kipling is from Mars.)
Here's the other thing about the word "if." It usually describes something that isn't real, or isn't real yet. It points to something could potentially happen, or potentially exist. Think of it like this: if somebody says to you "if you go to the store, you can buy soda," they are saying "hey, you haven't gone to the store, and you might not go to the store, but if you do go, then you can get some soda." So, the word "if" usually describes something that doesn't yet exist (you haven't gone to the store yet), but also implies that some other things will happen: you will be able to get soda once you're there. (We're going somewhere with all this—just bear with us.)
This is the basic idea of Kipling's poem. It is 32 lines of things that the speaker's listener hasn't done yet (learned to be strong, patient, wise, etc.). If he does them, but only if, then he will basically have possession of the entire world (a metaphor for power, a fulfilling life, and other things). What's more, the listener will be a man. In short, the title tells us that "If" is a poem about how certain things must be done. Only if those things are done, will certain other things happen.
And here's one more little thing to consider: We never find out what will happen if the speaker's addressee does everything he's supposed to, unless we read to the end of the poem. In other words, only if we read all 32 lines do we learn what will happen if the listener does everything he's supposed to do. Like the poem's addressee, we too have to do our part if we want answers.