Active voice describes a sentence where the subject performs the action stated by the verb. In passive voice, the subject receives the action. Passive voice is not grammatically wrong, but active voice is generally preferable on the ACT.
Examples: ACTIVE: The manager of the toy store put in a huge order for the new stuffed unicorns that were rapidly flying off the shelves. PASSIVE: A huge order for the new stuffed unicorns that were rapidly flying off the shelves was put in by the manager of the toy store.