This poem is about resilience and determination when facing oppression. This is a general theme but Angelou is also specifically talking about the historical oppression of African-Americans. In the poem, she shows a determination to resist being a victim of this oppression. No matter how she (the speaker) is portrayed in history books, she will "rise" (live, fight, resist). Even if she is forced to live a poor life, she will act as though she is rich ("oil wells"). She will "rise" with the certainty that the sun and moon both rise.
She challenges those who would oppress or hold her down. Rather than acting defeated, with "bowed head and lowered eyes," she will laugh and even be haughty or sassy. This is in defiance of the oppressors.
She ends the poem with
"I am the dream and the hope of the slave. / I rise / I rise / I rise."
Although no longer a slave, she still faces racism and oppression. Instead of wallowing in frustration resulting from this oppression, she endeavors to defy it and live a happy life. The penultimate stanza notes the difficult history her ancestors had to endure. She responds to that past with that same empowering refrain of rising.
Out of the huts of history's shame
Up from a past that's rooted in pain
I'm a black ocean, leaping and wide
Welling and swelling I bear in the tide.