The sentence active is: I keep the mayonnaise in the refrigerator.
The sentence passive is: The mayonnaise is kept in the refrigerator (same thing as the question)
If the subject is performing the action, then the sentence is in an active voice. However, if the subject is simply receiving the action, then the sentence is in a passive voice. A helpful tip is to add "by zombies" after the verb. It helps a ton. If you can insert "by zombies" after the verb then it is passive.