What was meant by the quote "I don't know, but they did it... Good night"
Atticus says this in response to Jem’s question, “How could they do it, how could they?” which, the text says, Jem asks “bleakly” because he is very upset about the jury’s decision to find Tom guilty in the face of all evidence to the contrary. Atticus means that injustice is common, that many times people will make the easy decision instead of the right decision, and that only children have the sensitivity to understand this enormous gap between right and wrong, and because they understand it, be sad and lament such human weakness.
When Atticus loses the case, Jem is upset and asks him how this happened. He responds with this quote acknowledging that racial hate is not just going to disappear. He knows how unfair the trial was, and he knows that this will happen again. The actual quote is, "I don't know, but they did it. They've done it before and they did it tonight and they'll do it again and when they do it - seems like only children weep." He is in a more positive frame of mind the next day-always fighting for equality.