A whole piece of writing should hang together, or, as one student put it, make sense. We sometimes describe the speech of someone who is excited or crazy as "babbling incoherently." What they're saying probably makes sense to them, but it isn't coherent for the listener because they're not being told the whole thing in a reasonable order and with all the pieces of information they need. This is a common difficulty for writers. Having someone else who can be objective read your writing, or getting some distance from it yourself, may help. Other kinds of coherence problems usually have to do with focus or organization, or both. Coherence problems can sometimes stem from mechanics as well: for instance, when the writing isn't consistently in the same tense, or changes from singular to plural, it is grammatically incoherent.