In English, all sentences are in either "active" or "passive" voice:
active: Werner Heisenberg formulated the uncertainty principle in 1927.
passive: The uncertainty principle was formulated by Werner Heisenberg in 1927.
In an active sentence, the person or thing responsible for the action in the sentence comes first. In a passive sentence, the person or thing acted on comes first, and the actor is added at the end, introduced with the preposition "by." The passive form of the verb is signaled by a form of "to be": in the sentence above, "was formulated" is in passive voice while "formulated" is in active.