What Atticus means in this statement is that when someone uses fould words or offends someone else, it indicates that the person is rude, disrespectful or mean. Therefore, it allows you to learn a lot about the speaker in question. However, it says little about the person being addressed, because the words can only affect them if they allow them to do so.
An example of everyday life can be when people tell you that it is ridiculous for you to try out something new. When someone says this, it shows that they are prejudiced, small-minded or fearful, and that they want to affect you. However, if you try new things anyway, then the words do not affect you and you are not allowing them to have any impact over you. The words can only hurt you if you allow them to do so.