There's no way to get a good summary in the kind of space we have here. You should read the book.
There are two story lines going on in the book. Both have to do with how you should treat people who are weaker or different.
In one story line, the main characters are obsessed with a guy called Boo Radley who is the town monster (everyone thinks he's a monster and he hasn't been seen in years).
In the other story line, a black man named Tom Robinson is on trial for raping a white woman.
Tom loses his trial and dies. Boo Radley turns out not to be a monster.