Well, it's the title of the poem, so that's got to be important, right? Right. It totally is, and in a couple of different ways. First of all, it's a historical truth – Trujillo ("El General") really did make Haitians try to pronounce the word "parsley" in order to determine whether or not they lived or died. On a more purely symbolic level, the word is representative of a whole legion of crimes and atrocities committed over simple, insignificant differences. (Skin color, anyone? Political preference? Religious beliefs?) The historical event, and thus the parsley itself, becomes representative of a kind of horrible arbitrariness – people hate for no reason, and they'll kill for no reason.
Thus, the parsley is not so much a plant in this poem. Instead, parsley represents the fact of difference and the (perhaps fatal) power of language. After all, the last line of the whole poem is "for a single, beautiful word." And we know exactly what that word is, and the weight of it is just huge.