Boo Radley appears to be like some monster according to Jem:
Boo was about six-and-a-half feet tall, judging from his tracks; he dined on raw squirrels and any cats he could catch, that's why his hands were bloodstained – if you ate an animal raw, you could never wash the blood off. There was a long jagged scar that ran across his face; what teeth he had were yellow and rotten; his eyes popped, and he drooled most of the time. (1.65)
Of course, Jem is exaggerating, but Boo is strange. Although Boo Radley was considered strange, he seemed to care about Scout and Jem. When Jem lost his pants at Boo Radley's house, they had torn on the fence. When Jem went back to get them, they were folded. Also, the pants had been sewn. It appears that Boo Radley is trying to take care of Jem. It is evident that he cares. Jem is puzzled in the care that Boo showed. He describes the way he found his pants:
When I went back, they were folded across the fence...like they were expectin' me...And somethin else--Jem's voice was flat. Show you when we get home. They'd been sewed up. Not like a lady sewed 'em, like somethin' I'd try to do. All crooked.
Boo appears to be very caring and genuinely concerned about Jem.
Again, by the end of the story, Boo fights and kills Bob Ewell while trying to protect Jem and Scout. Sheriff Tate protects Boo, saying Ewell fell on his own knife.
Atticus believes Jem killed Ewell in self-defense, but Tate makes him realize that Boo Radley actually stabbed Ewell and saved both children's lives. The men agree to claim that Ewell fell on his knife in order to save Boo the spectacle of a trial.
Although Boo is considered strange, he shows his human kindess when he protects Jem and Scout. Scout sees a human side to Boo while standing by Jem's bedside:
Boo saw me run instinctively to the bed where Jem was sleeping, for the same shy smile crept across his face.
Boo is extremely shy and introverted, but he is caring. He saves Jem's life and Scout's life. The statement that Harper Lee is making is that we should not judge a person until we really get to know that person. Boo Radley instinctively protected Jem and Scout. He is not some monster afterall.