When you use the passive voice, the subject is acted upon by the verb. In other hands, the action tends to be done to the subject.
Sarah was followed by a ominous shadow.
Sarah, the subject, is being acted upon.
The active voice describes a sentence where the subject performs the action stated by the verb. It is more clear and objective as well.
An ominous shadow followed Sarah.
Here, the subject, An ominous shadow, is acting upon Sarah.
