What does Atticus mean when he says to scout, "the one thing that doesn't abide by majority rule is a person's conscience?"

"The one thing that doesn't abide by majority rule is a person's conscience." This is a pretty famous quote. It is one of the larger themes of the book. Atticus means that although the majority may agree, each person knows in their heart what the truth is. Each person has the responsibility to step up and do the right thing. This is especially poignant when we consider the Tom Robinson trial.