This was a poem that Rudyard Kipling wrote as a way of "welcoming" the United States to the club of imperial nations. He is warning them about what their experience will be as an imperial power.
The main point of the poem is that imperial powers face a thankless task. Kipling argues that the people who go out to imperialize will have a difficult task. They will need to "wait in heavy harness," meaning that they will have to work very hard. But even though they work, it will seem to be for nothing. The people of the imperialized countries will be so inferior that they will ruin everything the imperializers have done. As Kipling says, the imperializers will
Watch sloth and heathen Folly
Bring all your hopes to nought.
In addition, they will be unappreciated. The people will not bless them for what they are doing but will instead hate them. What they will get for their efforts is
The blame of those ye better,
The hate of those ye guard--
Thus, the meaning of this poem is that it is hard to be an imperial power. Kipling is saying that imperial powers have to work hard for people who will not appreciate it and will ruin what they have been trying to achieve.