If you can dream---and not make dreams your master; If you can think---and not make thoughts your aim,
At the beginning of the second stanza, we encounter a very familiar train of thought and a very familiar literary technique: anaphora.
Yes, the speaker is back at it with more of the "if you do X, and if you do Y" business.
Here, he tells his listener that, if he can dream but not become a slave to his dreams, and if he can think but not make thoughts his aim…
Okay, so if he can, then what? We still have no idea what follows the "if," so let's just keep moving along.
The point of these lines is twofold. On the one hand, it's great to dream, but sometimes dreams can get just a little carried away.
For example, you might love ice cream more than anything. In fact, you might love it so much that, when you're starving, you dream about how nice it would be to have about ten gallons of it.
Sure, it sounds like a great idea, but it isn't: tons of sugar, tons of fat, lack of nutrients, a major stomach ache, etc., etc.
On the other hand, dreams are exactly that: dreams. They're not "real" in any physical sense. They are imaginary things, fantasies, mental ideas—something like that.
The speaker is saying that it's great to dream and to think about things, but at the end of the day, you have to focus on the matters at hand.
Let's say you've got a huge Algebra test tomorrow. And by huge, we mean massive, gigantic—the most important exam of your algebraic life.
While studying for this uber-important test, you start thinking about how when you grow up you wanna drive a Ferrari.
Then you start thinking about how you wanna move to Hawaii, and how you wanna spend every day at the beach.
Before you know it, it's bed time and you're not ready for your test yet.
By focusing too much on your thoughts and dreams—by making them your "aim" instead of the imminent exam—you've clearly done yourself a disservice, and perhaps jeopardized your grade.
And by jeopardizing your grade, you've just jeopardized mom and dad's willingness to buy you that nice steak dinner.
The speaker isn't saying, "Hey, don't daydream"; he's reminding the listener that practical matters must also be dealt with. The end result of obsessing over those other things can have a whole lot of negative consequences (as in our example: bad grades, no steak dinner, etc.).
We could sum up these lines in the following way if we really needed to: "Listen bro, if you can dream and think, but not lose sight of the tasks at hand, then [um… we don't know what goes here yet]."
Bottom line: it's important to stay focused, and to maintain control of your sometimes super-busy imagination.
If you can meet with Triumph and Disaster And treat those two impostors just the same:
The speaker now starts talking about a personified version of Triumph and Disaster.
If the listener can manage to meet with both success ("Triumph") and misfortune ("Disaster") and act the same way in each case ("treat those two impostors just the same") then...
Okay, we're just going to start leaving these little summations as a dot dot dot because it seems pretty clear at this point that we're not gonna learn the answer until the end.
Anyway, why are Triumph and Disaster both considered "impostors"? It sure seems like we should get to the bottom of this odd little quip.
We're guessing that it has something to do with the fact that both success and failure are misleading. Neither one lasts forever.
Success inevitably fades, and disasters always work themselves out.
In other words, they are both impostors because, well, they are short-lived.
Think of it like this: Success makes you happy. Disaster? Not so much. After some time, the emotions evoked as a result will go away.
This isn't always, strictly speaking, true—but you get the picture.
The point the speaker is trying to make is that one shouldn't get overly excited about success, or overly upset about disappointment.
He doesn't come right out and say this, but based on all the other stuff he's been saying about maintaining composure, being patient, and all that, it seems like a logical assumption.
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;
This time, the speaker tells his listener that if he can "bear" to see the truths he's spoken misinterpreted ("twisted") by knaves (jerkfaces) who use it to deceive foolish people…
And if he can watch everything he's ever dedicated himself to shattered, and yet still manage to pick up the pieces… then something will happen.
If we know one thing at this point, it is that we don't know what will happen. So let's just continue picking these lines apart.
A "knave" can mean a number of things. While it can sometimes be one of these, it is also a general term for a rogue, or a rascal, or a conniving evil guy, or what have you.
You can speak all the truth you want, but there will always be rascals around who will twist it, not literally, but metaphorically, to mean whatever they want and use it for their own ends.
"Things you gave your life to" is very general, but we can't help thinking it refers to something like a "cause" or an "idea" or a "career."
Sure, it could refer to that ant farm you've been working on for 2 years, but it's supposed to be something just a tad grander than that (unless you've got just a killer ant farm on your hands).
What those "things" are, however, is less important here than the ability to build those things again, even if the skills or desire one used is old, tired, and worn-out.
This is all very metaphorical, mind you, so we should probably try to make it a bit more concrete.
Let's say that you spent your entire twenties building a career as an attorney—you went to college, then to law school, then took the bar, and then got a job at a killer law firm.
After two years, the firm goes bankrupt (a miracle, given how much lawyers charge), and you're looking at the ashes of a career.
All may seem lost...
… but it isn't. You're tired, the bar nearly killed you, and so did that job search. But hey, you're still alive, and that desire to build a career as a lawyer is still there, even though it's a little "worn out."