It's the same as the first line of the poem, but with a little difference. The verb has changed from "sing" to "am."
Now, we know that your 9th grade English teachers probably told you that the verb "to be" is passive and bad and should be avoided in favor of more active verbs. Like "sing"! But in this case, "am" (which is just the present tense of "to be") makes this sentence way more forceful.
Why is that? Well, in this case, to "sing" something is kind of like a slightly more elaborate way of talking about something. But to full-on "be" something? That's a bigger deal.
At the end of this poem, the narrator is asserting his own existence, not "just" singing about something.
Moreover, he's claiming that he "is" America. A whole country! Impossible, you say. Well, true. But again, it's a poem, so we've got to think on a metaphorical level.
To "be" America, then, is to be representative of America. A country settled by a whole ton of different kinds of people, of all nationalities, backgrounds, cultures – you name it. The United States as a country has always been a diverse nation.
So as a member of a oppressed race, the speaker is asserting that he, too, belongs in America, is a part of it and is integral to its very existence. Just as much as anyone else.