Explain the causes and effects of the great migration

Great Migration was the movement of African Americans out of the rural areas of Southern United States to the urban Northeast, Midwest, and West. It occurred roughly between 1910-1970. The cause of this migration was not the usual one (where execution and politics are involved), but it was caused by Blacks finding a new one apart from their economic and social base in America. The effects were profound. It drained most of the Black population in the South. Cities in the north became of center of the new movement known as the black culture, it has enormous effect on both music and politics. Black metropolis develop an important infrastructure in various areas of social and political spheres, like for example jazz clubs. 

What does this mean ? ?