As Scout stands on the Radley front porch in Chapter 31 of To Kill a Mockingbird, she finally understands something Atticus said to her earlier.
What is it? And how has she come to understand it? I don't get this at all. How does she understand what Boo Radley is going through by standing on the Radley porch? What is the writer trying to convey? Why is Boo always inside the house?
Scout's dream finally comes true when she actually gets to see Boo Radley in the flesh for the first time. She had long come to understand that Boo was not the ghoul she and Jem had first assumed, but she certainly couldn't have imagined that he would eventually save her life. After she escorts Boo home and watches him close the door behind her, Scout takes a long look at her neighborhood from a different perspective--both literally (she had never been on the Radley porch before) and figuratively. Using Atticus' dictum that
"You never really understand a person until you consider things from his point of view--until you climb into his skin and walk around in it."
Scout does just that. She pretends that she is Boo, keeping watch over the neighborhood and reliving events that he must have seen over the past seasons. She stands in his shoes and sees the neighborhood through his eyes, imagining what he must have been thinking when he peeked out through the shuttered windows or stood on the porch when everyone else was sound asleep. She comes to understand that Boo wasn't so different: He was a neighbor who viewed the nearby surroundings like any other man, albeit one who preferred the safety of his solitary house.
The last part of your question, "Why is Boo always inside the house?" can be mostly answered from the telling of the neighborhood legend in the middle of chapter 1. Arthur Radley, (Boo's real name) while a teenager, had fallen in with the wrong crowd. One night, he and some Cunninghams "backed around the square in a borrowef flivver, resisted arrest by Maycombs ancient beadle, Mr. Connor, and locked him in the county courthouse."
The boys were charged with disorderly conduct, disturbing the peace, assault and battery, and using abusive and profane language in the presence and hearing of a female. The probate judge decided to send the boys to the state industrial school-- not a prison. But Arthur's father asked the judge to release Arthur to him. "The doors of the Radley house were closed on weekdays as well as Sundays, and Mr. Radley's boy was not seen again for fifteen years."
The account goes on to describe what we would call a disfunctional family, child abuse, and a number of personality and social disorders, and mental illnesses.
The narrative leads in to Dill's hatching the plot to draw Boo out into view.