Black people were originally brought from Africa to America during the 17th, 18th and 19th centuries. They were forcibly transported across the Atlantic in slave ships (in which many died) and sold as slaves to work on sugar and cotton plantations in the Caribbean and the southern states of north America. They had no rights and were seen by their white owners as little more than animals or machines.
Even after the abolition of slavery in 1865, the blacks were still almost powerless. The whites had too much to lose to allow blacks any rights. Nothing was equal: blacks had the worst of everything while whites had the best.
Such was the hatred of blacks by the whites - especially during the Great Depression of the 1930s, when money was tight for everyone - that it was common for blacks to be lynched [lynching: Executing someone (usually by hanging) without a legal trial. ] or hanged by a mob well into the 20th century - so there was good reason for Atticus to sit outside the jail to protect Tom Robinson before his trial.