Sentences can be written or spoken in the active or passive voice. In the active voice, the actor of the sentence acts upon something or someone. In the passive voice, the actor is acted upon and the verb "to be" is used. We are most accustomed to active voice, where we can answer the question 'who does what to whom?' in that order.
The passive voice often leaves out the 'who' or changes the order in which it appears.
Active Voice: The twister left a path of destruction through the town.
Who? The twister. Did what? Left a path of destruction. To whom? The town.
Passive Voice: A path of destruction was left by the twister.
or: A path of destruction was left.
Writers should usually use the active voice. It is considered a more powerful and straightforward form of expression. The active voice also uses fewer words to convey the same message. The passive voice is often used when the writer wants to be less direct, such as when avoiding assigning blame or responsibility, which is not a common priority in academic writing.