The meal was cooked by my mother and grandmother.
In a passive voice sentence, the action is done to the subject rather than by the subject. Remember that the time in which the action takes place does not determine active or passive voice: both of these sentences have past tense verbs. Writers generally want to avoid passive voice because it tends to weaken writing or make it less impactful. One hint for identifying passive voice is looking for the word by in the sentence (although a sentence can still be in the passive voice without by in it).