you never really know a man until you stand in his shoes and walk around in them

This sentence refers back to another sentence Scout says in chapter seven: 'I tried to climb into Jem's skin and walk around in it'. Back in chapter seven, Scout wasn't able to crawl into Jem's skin and understand him, but in this final chapter of the book, she is able to grasp who Boo Radley is as a person. This shows Scout's growth throughout the novel and emphasises the point that she is getting older.