Prose is so-called ordinary writing — made up of sentences and paragraphs, without any metrical (or rhyming) structure. If you write, I walked about all alone over the hillsides, that's prose. If you say, I wondered lonely as a cloud/that floats on high o'er vales and hills that's poetry. See the difference? (Let's not get into prose poetry!) From prose we get the term prosaic, meaning ordinary or commonplace, or lacking the specially delicacy and beauty of its supposed opposite — poetry.