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

He means that your own conscience must tell you what is right or wrong, and it does not matter how many others agree or disagree with you. As Atticus explains to Scout, this is why he is defending Tom Robinson, even though most of the town thinks he is wrong for doing it

the reason some people sacrifice their own life for another person's life could be that they are in love with them and would do anything to keep them safe a protected even if that means risking their life in the process

citizens are given 25 years of life to prove their worth to society; if they fail to do great things