Parallel sentences basically mean balanced sentences. A pattern of words is used to show that multiple ideas have the same priority level. The words coordinate. This can happen at the word, phrase, or clause level.
For example:
(Parallel) To be a skilled musician, you must arrive at lessons on time, practice your instrument and learn different songs.
(Not parallel) To be a skilled musician, you must arrive at lessons on time, practice your instrument and be sure you are willing to learn different songs.