In passive voice sentences, the subject is acted upon by the verb rather than doing the action. The basic structure uses a form of "to be" (am, is, are, was, were) plus a past participle. For example, "The cake was baked by Maria" shows passive voice because the cake receives the action of baking.
This construction often includes the word "by" to identify who performed the action. However, the doer can be omitted when unknown or unimportant, as in "The window was broken."