In the poem, Dove writes, "The courtesy/of one seat/among many/was denied her." This means that Rosa Parks was not given the respect that she deserved as a human being. She was denied the right to sit in a seat that was rightfully hers, simply because she was black.
The rest of the poem goes on to describe how Rosa Parks' act of defiance sparked the civil rights movement. Dove writes, "That courtesy/of one seat/among many/we now claim as our right." This means that all people, regardless of race, deserve to be treated with respect and dignity. Rosa Parks' act of courage paved the way for all people of color to demand and receive equal treatment.
The title of the poem, "Rosa," is also a reference to the way Rosa Parks is treated with respect and honor. She is called "Rosa," which is a familiar and affectionate name, rather than by her full name, "Rosa Parks," which is more formal and impersonal. This shows that the speaker of the poem is addressing Rosa Parks as a friend and an equal.
Overall, the word "courtesy" in the poem "Rosa" by Rita Dove refers to the act of paying respect or honor to someone. Rosa Parks was denied the courtesy of a seat on a bus, but she sparked the civil rights movement and now her "courtesy" is now ours.