Mockingbirds are said to signify innocence, fragility and altruism because it is harmless and simply sings, but it is hunted and mindlessly killed by children and hunters. In the same way, Tom Robinson represents an innocent mockingbird that, although he does no harm, he is ignored and killed, just like a mockingbird.
Despite the fact that he is crippled, Tom Robinson is a loving husband and father who juggles the demands of his family and his job, while trying to establish himself as an upstanding and hardworking member of society.
The author likened Tom’s murder to the meaningless slaughter of mockingbirds.