What does it mean when a white man calls a black man uncle?

This comes from the book "Uncle Tom's Cabin" by Harriett Beecher Stowe. In the book, Uncle Tom is a black slave and a devout Christian, who is sold to a horrible plantation owner named Simon Legree just days before he was to be set free by his previous owner (that owner died and his wife sold Tom instead of setting him free). His faith in God is tested, but he does not waiver his spiritual beliefs, and on his deathbed, he forgives his tormentors.

During the Civil Rights Movement of the 1960's, Uncle Tom was not remembered so much as a devout Christian man as he was a slave who just rolled over to please his masters. Many blacks saw his character as accepting of a position subordinate to whites, a quality not shared by those crusading during those volatile times. Hence, the term "uncle" came to mean a black man who accepts inferior treatment by whites.