"Ballad of Birmingham" is, indeed, a ballad, or musical poem. Ballads are often about heroes or sad tales, and this poem is no different. As well, ballads are often well-known—even by those who aren't poetry buffs. It's not a coincidence that Randall chose this form, since he wanted to spread the truth—and the horror—of this story as far and as wide as he could.
So, that explains the ballad part of our title. Now for the location: In the early 1960s, Birmingham was the setting for numerous marches, protests, and meetings about civil rights for African American citizens who faced daily segregation because of the Jim Crow laws in effect in the South. In retaliation, white supremacists often resorted to violent means. This poem is a sad tale of an actual 1963 bombing that happened in a church in Birmingham, ending the lives of four young girls that day.