When you join two or more ideas—in the form of words, phrases, or even clauses—in a sentence (usually with the help of a conjunction), the ideas you connect must be parallel. In other words, the grammatical structure of each one must be the same.
The basic parallel structure rule is that the things in a list should be in the same grammatical form. If you are listing three things, the construction of that list should be noun, noun, noun, or verb, verb, verb, or gerund, gerund, gerund, etc. Any inconsistency within the list is an error in parallel structure.