As per the grammatical rules of English, in sentences, there is a
subject: who is doing something a.k.a. the actor.
and an object: to which something is done a.k.a. the receiver.
In the active voice, the subject acts upon the object and in the passive voice, the object is acted upon by the subject. The difference occurs in the emphasis given to which part of the sentence.
So, if the emphasis is laid on the subject, i.e. Garry played the guitar, it is an active voice and if the emphasis is laid on the object, i.e. the guitar was played by Garry, it is a passive voice.