What does Atticus mean when he says..."You never really understand a person until you consider things from his point of view- until you climb into...

This is plain as day. Atticus has wanted his children to look at life from Boo's perspective, from a Negro's perspective, and from their peers perspectives throughout this story.

What may be confusing here is the idea of climbing around in someone's skin. I think it would be easy to just put someone's shoes on. You would feel that the shoes were a little tight or a little loose. And you then could give the shoes back.

To walk around in someone's skin would mean adopting their life characteristics, the things they are persecuted about, praised for, expected of, and stuck with. You can't just quickly adopt someone's life traits. This would take deep analysis and consideration. Scout only ever gets this when she stands on Boo Radley's porch and imagines what it might have been like to watch her over the years.