From which latter sense you might infer that prosody is mostly about meter and rhythm and accents, but it is also informally used to encompass stanza forms, rhyme schemes, and similar defined aspects of form. More strictly, though, it's about the meter. And as a sometimes poet who writes mostly in forms, it's a subject near and dear to my heart. The word was borrowed in the mid-15th century from Latin prosōdia, the accent of a syllable, Greek prosōidia, accent, originally song set to music, from pros, towards/to + ōidē, song, the origin of ode. So nothing to with prose, which is purely Latin root (from prōvertere, to turn forward, as in what one does when making a speech -- in prose).
Milton's prosody is nothing short of astonishing, but I still can't stand Paradise Lost as poetry.