What, according to Kipling, is the "white man's burden"?

According to Kipling, the white man's burden is the need for white, "civilized" nations to travel abroad and impart their values and culture to other nations. The poem, therefore, is a defense of imperialism. This is made clear in the first stanza of the poem when Kipling talks about white men sending their sons abroad to serve their "captives' need."
For Kipling, this burden is a necessary one because people living abroad are in urgent need of civilization. He calls them "half devil and half child," for example, and suggests that they are "wild." Moreover, they suffer from "famine" and "sickness" and, therefore, are in need of the white man's help.
Although this task is a necessary one, Kipling argues that it is a "burden" because people will not appreciate it. He talks about "ungrudged praise," for instance, and "thankless years." However, Kipling believes that imperialism is so necessary that it is worth suffering the judgment and criticism of white peers and colonized citizens alike. It is, in his view, the only way to civilize foreign nations and to increase their cultural and economic worth.

According to Kipling, the "white man's burden" is a call for predominately white nations to send their best and brightest white males to uncivilized lands to spread Western civilization and culture to the "sullen peoples." Kipling presents the concepts of imperialism as a just and honorable goal to civilize the apparent uncivil natives around the world. Kipling challenges civilized nations with Western ideals to humbly embark on a journey of imperial conquest, which apparently benefits the foreign people living in uncivilized territories. Kipling describes the duties of the "white man's burden" by encouraging imperialist nations to promote peace, educate the natives, and feed the starving. Kipling mentions that the white men should toil as they build infrastructure in the foreign lands and cautions them against becoming lazy. He also informs the imperialists that their work will be difficult and encourages them to persevere even if they do not receive praise from the natives they are civilizing.

"The white man's burden," according to Kipling, is the civilization of the supposedly uncivilized peoples of the English colonies. Each stanza highlights a different component of the "burden."
In the first stanza, the "burden" is traveling to a foreign land to serve the native peoples. The second stanza emphasizes the need to educate the foreigners with Western philosophy, which will ultimately "work another's gain." According to the third stanza, the "burden" is to fight "heathen" evils like famine, even in the face of discouragement. The "burden" is of servility in the fourth stanza and of thankless labor in the subsequent stanza. (Apparently, the natives will not realize what exceptional aid it is they are receiving.) The final stanzas conclude with the clarion call to take up the burden, regardless of the hardship.
Since its publication, Kipling's poem has received a great deal of criticism, owing to its assumptions that white (Western) culture supersedes all others and that said cultures are ignorant and subsequently indebted to whites for their intervention. The same year (1899), H. T. Johnson responded with the poem "The Black Man's Burden."
http://historymatters.gmu.edu/d/5476/