Parallelism is a term that describes the use of similar elements used in grammatical or literary constructions. Parallelism is used to make comprehension easier, provide rhetorical impact, and create memorable phrases in writing.
It’s also a concept that can work on multiple levels, both grammatically and rhetorically.
Give a man a fish and he will eat for a day. Teach a man how to fish and you feed him for a lifetime.
Basically, parallelism uses some sort of repetition to create a better flow or more memorable phrasing in a written work. The repetition is not always of individual words. Instead, it can be reusing a particular grammatical construction repeatedly