In 'he is a prisoner of memory', I don't understand the meaning; could anyone help me?
PS I'm sorry to say that this is recorded in a dictionary as an example sentence, so there is no context.
He cannot, or will not, forget certain events. It could be that he lost all of his money on a gamble and he keeps thinking about it. He cannot move forward in life because he cannot live in the present; he is , as we say, stuck in the past.