Parallel structure uses the same grammatical form for items that do the same job.
Matching forms
Not parallel: I like reading, to swim, and cycling.
Parallel: I like reading, swimming, and cycling.
Equal items
All nouns, all -ing, all infinitives, all clauses. She is smart, kind, and funny — not smart, kind, and a comedian.
I like reading, to swim, and cycling. — no
I like reading, swimming, and cycling. — yes