There is no fancy or formal form or meter in this poem, but that doesn't mean there isn't a very definite structure and intentional pace. When you look at the poem on the page it might not seem like much. It looks short (which may come as a relief to some of you) and kind of skinny. That's because it is. It's only twelve lines long, made up of four stanzas that are three lines each. Those lines are all of pretty even length (you don't see any stretching all the way to the right margin while most are hanging tight by the left), and they're short too. What Dove is doing with this structure is similar to what she was trying to achieve in the title. She's making a simple, quiet poem into something powerful, just like Parks herself who was a humble, normal person that did something incredible.
What it also does is slow things down significantly. Rather than cramming sentences in a single line, she stretches them over a few lines and that gives us a chance to pause at the end of each line to soak it in: "That trim name with / its dream of a bench / to rest on" (4-6). We really get to absorb this event, even meditate on the small, often overlooked aspects of it because of the length and pace of the lines.
Dove's use of enjambment helps this effect along, as well, by doing two things. The first effect of breaking the line off before the end of a sentence is that it allows the reader to mediate on key elements of that sentence which are set apart on their own lines, like "its dream of a bench" (5). At the same time, the reader is propelled through the poem with this technique. The cut-off line is like a mini-cliffhanger, drawing the reader along into the next line.
Often, what you get in that next line is another fragment. The final sentence in the poem isn't even a sentence: "That courtesy." And yet it has a ton of possibility in it, in terms of meaning. That's what this poem is trying to achieve in its form and structure—small, patient moments of reflection (much like the gesture of refusing to give up your bus seat) that create a powerful collective kaboom (like this poem, or the Civil Rights Movement).