Parallelism is achieving balance in sentences. When you present similar points in a sentence, those points should have similar structure. This means that if you have a list of adjectives, that each word you use in the list is an adjective. If you are presenting a series of verbs, the verbs all use the same form and tense. When sentences aren`t parallel they are often very awkward and difficult to read. Readers should not be distracted from the meaning of your writing because your sentences aren`t understandable.
Parallelism is achieved by finding the list within a sentence and then checking to see what type of structure best suits your meaning. Sometimes a preposition can introduce a list of three objects; at other times, each object will need its own preposition. You make those decisions when you evaluate the sentence you are trying to make parallel.