What do you mean when you say the verb is in the active voice?

Active sentences have this basic form -- subject + verb + object.

The subject is the person or thing that does the action of the verb. The object is the person or thing that "receives" the action eg

James ate the cake.

In this sentence we can see who did the action of eat (ate is past tense) it was James and the thing he ate was the cake (object).

In passive sentences there is often no subject and the verb phrase has this form -- be + past participle The object is also in the subject position eg

The cake was eaten.

In passive sentences if you want to say who or what did the action of the verb then you do it by adding -- by + noun/noun phrase at the end of the sentence eg

The cake was eaten by James.

Passive sentences are often used for writing about a process where it is not important who or what does the action eg

The letters are taken to the sorting room. There they are put in a sorting machine.