Misplaced and dangling modifiers crop up frequently in a new writer's prose - usually the dangling modifier is the problem. These are errors of sentence structure. A misplaced modifier involves a phrase misplaced in the sentence, too far from the object it is supposed to identify. An example of a misplaced modifier:
Julie put the horse in the barn that was her childhood favorite.
The horse is her childhood favorite, but barn was inserted between the correct object and its modifying phrase, making it sound as if the barn was Julie's favorite. To correct:
Julie put the horse, her childhood favorite, in the barn.
Now that the misplaced modifier is back where it belongs, beside the word it is supposed to modify, everyone's clear which item is her favorite. Now for the dangling modifier, the more common error. These usually involve an -ing word, and the identifying object is left out of the sentence, skewing your meaning. For example:
While riding the horse, the sun shone brightly.
The sun is the object of the sentence given in this wording, but we all know the sun wasn't the thing riding the horse - Julie was, and she's missing completely from the sentence. To clarify, you need to put Julie back into the sentence:
While Julie rode the horse, the sun shone brightly.
Julie's doing the riding in this wording, and it's clear that the sun isn't. Look for these mistakes in your writing, and you'll avoid having readers that stop to wonder what you're talking about. Keeping your meaning clear helps readers stay involved in your story, so they see the story, not its mechanics.
Side Note: Thanks to Ryan Field for his kind compliment on my Telling vs. Showing blog entry - glad to be a help!