When a verb has no object, we call it intransitive:
Shoes wear out.
When a verb takes an object, we call it transitive:
Running on concrete wore out his knees.
"His knees" is the object. "Running on concrete" is the subject.
We can "flip" a transitive verb in the active voice (see the sentence about "knees" above) and make it a passive construction:
His knees were worn out by running on concrete.
We can omit the agent (that which performs the action):
His knees were worn out.
Something wore his knees out, but we don't know what it was. Was he a fireman who climbed ladders all day? In this form, we're basically saying his knees are in certain state or condition, since the agent is unexpressed.