She cooked a meal -
this is in the ACTIVE voice. It means that she did the action. In this case, she cooked the meal
She was cooked a meal -
this is the PASSIVE voice (to be + past participle). It means someone else did the action. Often the 'agent', the one doing that action, can be omitted if it is not necessary. For example, maybe her mother cooked the meal.
She was cooked a meal by her mother.