What are the “facts of life” according to Atticus?

Atticus explains to Jem some of the "facts of life" in the segregated South where Maycomb is located. One of these facts is that between the ideal and the reality there is a grave difference when matters involve white people and black people.
When Atticus made his closing remarks at the travesty of a trial against Tom Robinson, he reminded the twelve men of the jury that a court of law in America is designed so that every individual is equal under the law. However, Tom was not treated fairly by the jury. For there was no hard evidence directly against him; the trial was merely the Ewells' words against the word of Tom Robinson. Moreover, what was discovered about Tom is that he has a virtually dead arm that hangs limply at his side, so it would have been impossible for him to have left marks on both sides of Mayella Ewell's throat from choking her, as Mayella had testified. Tom was convicted on only circumstantial evidence because "something came between...twelve reasonable men in everyday life." Atticus tells his son that these men could not be fair if they had tried in the courts of the Jim Crow South. These facts are "ugly, but those are the facts of life." (Ch.23)