When the word "burden" is right in the title, you know this poem is going to be talking about hard work. According to Kipling's speaker, though, busting your rear is a good thing. He's not about any easy outs. Work for him is a way to prove your worth. Well, let's be honest. It's a way for white men to prove their worth. Given that they're the ones being called on to do the work of "civilizing" the world in the first place, we're led to believe that our speaker already thinks pretty highly of these guys. Still, he'd think even more highly of them, if they'd just get their hands dirty.
Lines 1, 9, 17, 25, 33, 41, 49: The poem uses a metaphor in this refrain to pull a double technical whammy on us readers. The idea of a "burden" is central to this poem, which wants us to know that the act of taking over the world and forcing its people to behave like European white guys is… well, no easy task.
Line 3: The image of the "bind" here is one that suggests that white men should almost be shackled against their will to do this work. It doesn't sound like a pleasure cruise, that's for sure.
Line 5: Now these poor guys have to put on a "heavy harness." Again, the idea of difficulty and strain—along with an image of useful and productive effort (a harness, after all, can be used to pull all sorts of things)—underlines the speaker's insistence on hard work.
Lines 43-44: This work is tough, it's thankless, and—thanks to some personification of freedom here, it's apparently not something that you can take a break from. Hard work, always—that's our speaker's motto.