what is the meaning of the title phase "It's a sin to kill a Mockingbird"

This is because mockingbirds merely bring beauty and happiness into the world. At one point Miss. Maudie explains,

"Mockingbirds don't do one thing but make music for us to enjoy. They don't eat up people's gardens, don't nest in corncribs, they don't do one thing but sing their hearts out for us. That's why it's a sin to kill a mocking bird."

The title refers to innocent people like Tom Robinson and Boo Radley who, like the mockingbird, only wish to bring a little happiness into a harsh world.

"To kill a mocking bird" means don't harm anything unless it does it to you. You need to be nice to the innocent ones such as Boo Radley and Walter Cunningham. They are just trying to make peace in a tough time and in a rough world.