When using active voice, the subject performs the action noted by the main verb. (As one site put it, the subject is a do-er or a be-er.)
When using the passive voice, the subject is acted upon by another agent or an unknown something. (The subjects of passive sentences are not do-ers or be-ers.)
These are much better explained with examples:
Active: Brutus stabbed Caesar.
Passive: Caesar was stabbed by Brutus.
Active: The committee approved the new policy.
Passive: The policy was approved of by the committee.