Mockingbird: The mockingbird represents innocence.  Like hunters who kill mockingbirds for sport, people kill innocence, or other people who are innocent, without thinking about what they are doing.  Atticus stands firm in his defense of innocence and urges his children not to shoot mockingbirds both literally and figuratively.  The mockingbird motif arises four times during To Kill a Mockingbird.  First, when Atticus gives Jem and Scout air guns for Christmas and instructs them not to kill mockingbirds.  Second, when B.B. Underwood writes about Tom Robinson's death in his column.  Third, a mockingbird sings right before Bob Ewell attacks Jem and Scout.  Finally, Scout agrees with Atticus that prosecuting Boo for Ewell's murder would be like killing a mockingbird.

Boo Radley: Boo Radley represents fear.  Small town folks fear that if they act eccentric and fail to adhere to social rules they too will end up like Boo, isolated and remembered as a grotesque monster.  It is this fear that supports the social status quo and keeps individuals from standing up for that which they believe.  Until people can understand and accept Boo, as Scout does at the end of the book, they will always be stuck in a world filled with fear, lies, and ignorance.

Guns : Guns represent false strength.  According to Atticus, guns do not prove manhood or bravery.  Manhood and bravery come from a man's ability to persevere and fight using his wits, his heart, and his character.  Neighbors use and venerate guns to the detriment of developing their own personal strength.