Parallel structure means using the same pattern of words to give equal weight to two or more ideas.
Examples: CORRECT: At summer camp, I like to hike, swim, and play tennis. INCORRECT: At summer camp, I like to hike, swim, and playing tennis. CORRECT: Percival Lowell, who discovered the erstwhile planet Pluto, used both his advanced understanding of mathematics and his background in astronomy to back up his hypothesis. INCORRECT: Percival Lowell, who discovered the erstwhile planet Pluto, used both his advanced understanding of mathematics and he had a background in astronomy to back up his hypothesis.