Please explain the line 'Or walk with kings-nor lose the common touch' from the poem "If," by Rudyard Kipling.give detailed answer

As you probably know, this poem is all about what someone (the speaker's son, presumably) must do in order to become a man.  The line you cite gives two of the things the son must do.

The son must be able to mix with anyone.  He must think that he is good enough to "walk with kings" and he must act in a way that will make high status people like kings respect him.

But, at the same time, he must not get stuck up.  He must not "lose the common touch."

So what the speaker is saying is that, to be a "man" you need to be proud and self-confident, but you need to still be humble at the same time.