As the linked handout explains, "A modifier is a word, clause, or phrase that modifies, or further describes, another word or group of words in a sentence. A modifier is best placed directly next to the word or phrase for which it gives more information. When a modifier is misplaced, the reader is unsure about the meaning of the sentence."This means that most modifier mistakes can be corrected by moving the modifier next to what it describes, which might require some rewording of the sentence.
For more specific explanations and examples of various modifier mistakes, please see the linked handout: