(When writing works like math [again] …)
Parallelism is a concept that says the elements of a sentence should be grammatically balanced. (Remember how in math all elements in an equation must be in equivalent form?)
Not parallel: Mom wants me to buy a sponge, the hose, and wash the car. (the element “the hose” has no verb, unlike the other two phrases)
Parallel: Mom wants me to buy a sponge, find the hose, and wash the car.
Not parallel: I want to know where you’re going and when.
Parallel: I want to know where you’re going and when you’ll return.