Writing in the passive voice hides the subject (that is, the actor) of your sentence. The active voice keeps the subject of your sentence visible. In this sample student paper, Grammarly has identified eight instances of passive voice.

The sentence selected here begins,

Intelligence is a word that has been used by scientists…

The subject of this sentence is "scientists," since the scientists are performing the action of the sentence ("have used"). To rewrite this sentence so that the subject is visible (using the active voice), the student should write,

Scientists have used the word intelligence…

You can find more information about and examples of passive voice at Active & Passive Voice.