Rudyard Kipling's poem "If—" reflects aspects of traditional British stoicism, which is to say, the longstanding cultural honoring of repressing one's emotions or reactions in social situations. This is commonly referred to throughout the UK as maintaining a "stiff upper lip." This well-loved poem address Kipling's son, John, and encourages the boy to persevere despite the attitudes, actions, and behaviors of those around him who might have lower moral or emotional standards. In the first stanza, Kipling writes:

If you can wait and not be tired by waiting,

Or being lied about, don’t deal in lies,

Or being hated, don’t give way to hating,

And yet don’t look too good, nor talk too wise...

Here, he is advising his son to learn patience, to become an honest man, and to hate no one else; all of this must be done although the boy will likely encounter liars, haters, and people in a constant rush. The last line of this stanza also cautions him to allow his ego (via his public acknowledgment of his own "goodness" and "knowing") to act in equally as destructive ways. It is not enough to be good; one must be humble as well.

Exaclty. This poem is all about balance and perfecting one's attributes. "Don't look too good" means you're not putting on airs, not going to great lengths to outshine everyone. "Nor talk too wise" is along the same lines. It means you don't have prove your intelligence at every moment...you don't need to talk down to anyone. So basically, the poem is saying if you can do all those other things, without becoming egotistical, then you've made it.