We sent along the following advice: Use your search mode to look for prepositional phrases that can be deleted, by searching and, but, so, to, because and the like and making sure both parts are needed to make your meaning clear. Essentially, there's nothing wrong with the writing, when a passage tends to go on a bit too long. This is a matter of pace, not prose. Often such passages can be a bit redundant.
You have to tell the reader everything, but you only have to tell him once.
Even good writing that is too full of prepositional phrases, overused pronouns and adjectives, or where the action moves too slowly, can be tiresome to readers. So essentially what a publisher means, when she writes "tighten" is that I feel the work may be moving a little slow, but she can't put her finger on anything that is really wrong, so the usual thing to do would be to see if you can cut some of it out, without losing any of the meaning.