It is a form a verb takes to indicate whether the subject performs or receives the action.

These are the two types of voice.

It is used when the subject is the recipient of the action.

It is used to show that the sentence's subject is performing or causing the action.

The cake was made by her. 

I went to Korea last month.

Mark bought a new house.

At night, Karen ate a lot of snacks.

A lot of snacks were eaten by Karen at night.