What does the Roll of Thunder Hear My Cry poem mean?

Roll of thunder, hear my cry... White people listen to my Words

Over the water, bye and bye... Over the ocean where you stole me from over time

Ole man coming down the line... slave master keeping slaves chained together

Whip in hand to beat me down... slave owner beating slaves into submission and to lose all hope

But I ain't gonna let him turn me around... I refuse to lose my self-respect and dignity of self worth