The quote in full says
“If you just learn a single trick, Scout, you'll get along a lot better with all kinds of folks. You never really understand a person until you consider things from his point of view . . . until you climb inside of his skin and walk around in it.”
The message is quite simple: You have to be in someone's shoes in order to clearly appreciate what they are going through. One cannot assume that people are supposed to act in one way or another, or make this or that decision only because it is what we would have done.
Atticus is trying to explain to Scout that life is not about expectations but about understandings. People can change in front of our very eyes, or end up being someone we never saw coming. Yet, it is a fact of life, and it is what it is. Therefore, it is better to just try and consider the circumstances surrounding a person to clearly judge the character of the individual and, more than anything, accept the differences.