An example of a rock opera would be Queen's Bohemian Rhapsody. The instruments and style of the music is rock, but the music is exaggerated the same way that opera is. The term can be used loosely to describe over exaggerated rock music.
Just as 'opera' evolved over the centauries one could say that 'rock opera' is a further evolution.
Just like 'opera' it tells a story but a very modern setting, with rock instruments and...