When a sentence is written, it can either be written in active voice or passive voice.
Active voice is used when the subject performs the action stated by the verb.
For example: Tom changed the flat tire.
Passive voice is used when the subject is acted upon by the verb.
For example: The tire was changed by Tom.
In most cases, writing sentences in passive voice is discouraged because it can obscure the subject of the sentence and confuse the reader. It also creates a wordy and awkward sentence structure. Once you learn how to change passive voice to active voice, it's one less thing you'll have to edit.