What does it mean when Atticus tells Scout she could get along better with a person by crawling into that person's skin?

If she imagines what it is like to be in the shoes of that other person, she might be able to understand their ideas and motivations.

This shoe atticus trying to show scout that if she tries to see things from someone elses perspective then she will understand maybe why they do or say certain things. it shows that people she things differently because they have been brought up differently and helps scout not to judge people.