It's to do with the subject of the verb. Mary sells cakes. Here, Mary is the subject and she is doing the action of the verb, she is selling the cakes. This is an active sentence. In a passive sentence, the subject is on the receiving end of the action, the verb is acting on the subject. Cakes are sold. The cakes are being acted on by the verb, they are being sold. In life, if you are doing something, you are being active. If something is being done to you, you are being passive. I hope this helps.

The word stole is the past of "to steal". I steal, I stole, I have stolen. When we go from active to passive, we change the object of the sentence to become the subject. And we convert the active verb to passive by putting the verb to "to be" + the past participle (the 3rd column of the verb). Mary sells cakes. (Active present). The cakes ARE SOLD (present form of "to be" plus the participle of "to sell"). The robber stole it last saturday (active past). it WAS STOLEN (PAST form of "to be" plus the participle of "to steal").