My instructor wrote on my paper to be careful about using passive voice. What does that mean?

In a sentence, when the subject is acting out the verb (or action), the verb is in active voice. When the subject becomes the target of the verb (or action), the verb is in passive voice. The dog chased the cat is active, but The cat was chased by the dog is passive. Aaron cuts my hair is active, but My hair is cut by Aaron is passive. The stock market crash caused the Depression is active, but The Depression was caused by the stock market crash is passive. Jacob loves Kristin is active, but Kristin is loved by Jacob is passive. Get it?
In passive sentences, the subject and the direct object switch places. Passive sentences, by their nature, are less compelling than active ones, so you should avoid them wherever possible. To make your sentences strong, your verbs should be action-oriented.