In the basic subject-verb-object sentence (i) the subject does the action, (ii) the action is done by the verb, and (iii) the object receives the action. Of course, not every verb takes an object. The sentence"I think" has no object.
Subject-Verb-Object sentences are called SVO sentences. This is what we call 'normal' sentences. In these sentences, the actor is first identified (the subject), then we reveal the action performed by the subject (the verb) and then, if needed, follows the receiver of the action (the object). This is 'active voice'.
In contrast, 'passive voice' focuses on the action or on the receiver of the action. Either the object, or the verb comes first, but not the subject of course. You would omit the subject, or add it at the end if you really have to. At this point you need to be aware of this, no need study and learn and memorize. That will come later. Here are some passive voice examples.
The book was published. (active voice: Gutenberg published the book.)
The product was released. (active voice: Sony released the product.)
The official was investigated. (active voice: The police investigated the official.)
The pedestrian was run over. (active voice: The truck ran over the pedestrian.)
You get the idea? Here is the test for a passive sentence. If you ask 'by whom' and the answer is not the first thing in the sentence (the subject), it is passive. Let's try it out.
The book was published by the UN. (by whom?) By 'the book'? No. So, it's passive.
The UN (subject) published (verb) the book (object). (by whom?) By the UN? Yes, it is active.