“Always do right.”
Mark Twain

I think what mark is trying to say is that the key to greatness is doing right. Doing what is right and being nice and not expecting anything in return. When you do the right things, good things will start happening to you. Doing wrong may just bring you consequences. It wont lead you anywhere. So I think he’s trying to tell people to be the best they can be and to be a good person.