Please help me with the meaning of these lines from "The Negro Speaks of Rivers.""i've known rivers ancient as the world and older than the flow of human blood in human veins"
These lines are part of the poem's essence. The idea of an individual, a Black individual, being a part of the world historical stage so as to be able to "know rivers ancient as the world," helps to bring to light that Black culture and the culture of those from Africa have borne witness to the world's development. At the time in which Hughes is writing, many individuals of both Black and Caucasian ethnicities lacked the full understanding of the historical presence of Africans in world history. The idea that the knowledge of these rivers is "older than the flow of human blood" helps to bring to light that the knowledge of Africans about world history and their part in it is quite old, almost ranging to the first human presence in the world.
Basically, this poem is something of a "black pride" poem. It is a poem that celebrates the history and importance of black people. The lines you cite introduce this idea.
What the poet is saying in these lines is that he (and by extension black people) have been around as long as any humans. He is painting a picture here of great antiquity, of time long before history.
He is saying that black people were there from the beginning. This is a way of expressing pride in his people and the conviction that they have a history as long and important as anyone else.