Line 12 occurs in the second stanza. In this stanza, the speaker says he knows why a caged bird will beat its wings against the bars of his cage until they are bloody. The bird is tired of being imprisoned and wants to be free, even if the cost is painful. The bird is frustrated, the speaker says, because he cannot get free to swing on the bough of a tree. The bird also feels, after bloodying his wings trying in vain to get free, that

A pain still throbs in the old, old scars.

In other words, it is extra painful for the caged bird because he not only experiences his lack of freedom today but the pain of all the times in the past he has tried and failed to achieve freedom. These are his "scars." The pain becomes cumulative and worse the longer he is kept in his cage. Dunbar goes on to say that the bird's song is not joyful but a plea to God that he be set free.

Dunbar was a Black man who wrote this poem in the late 1890s, a time of deep racism and legal segregation in America. The caged bird is a metaphor for the way racism makes the speaker feel caged and imprisoned, with few opportunities in life. What makes it worse are the scars he has accumulated over time from trying to break free.

Through the metaphor of the caged bird, we as readers feel the frustrations brought on by racism.