In To Kill a Mockingbird, Scout has had a hard time getting adjusted to the challenges of being in school. Her teacher, Miss Caroline, repeatedly misunderstands her good intentions and punishes her: first for being an exceptional reader and then for speaking up on the behalf of Walter Cunningham. The final outburst of the day occurs when Burris Ewell gets scolded by Miss Caroline for coming to school unbathed, which results in Burris unleashing an abusive tirade against the poor woman.
Thus, when Scout comes home complaining about her day and asking her father if she has to keep going to school, Atticus tells her:
[I]f you can learn a simple trick, Scout, you'll get along a lot better with all kinds of folks. You never really understand a person until you consider thing from his point of view... until you climb in his skin and walk around in it.
This advice--which suggests that Scout be more empathetic and compassionate to those around her and that she should give others the benefit of the doubt--helps her forgive what has occurred at school that day. She realizes that Miss Caroline is a new teacher who is very young and inexperienced with the customs of Maycomb; she could not be held responsible for not knowing any better on her first day of teaching. Scout also realizes that she may not take out her disapproval on Burris or any other child who falls out of the realm of the "common people."
In the novel To Kill A Mockingbird, Harper Lee expresses many opinions through the voice of Atticus. With this expression, Atticus is saying that any reader cannot understand what another person is going through until you see the world through their lens or the way they look at the world. If you "climb into his skin and walk around in it", you experience what they experience in the same way as they do because you are seeing what is happening through their lens or through their experiences in life which have shaped them into the person they are. Truly understanding another person is far more difficult than most people believe, and this idea of walking a mile in their shoes or climbing "into his skin" makes you feel with the same feelings they do and understand being the outsider they are.