I've seen the film "Casablanca" so many times that I know a lot of it by heart.

know 是动词，意思是"知道、了解、熟悉"。 by heart 是一个固定短语，字面意思是"用心"，引申为"熟记、背诵"，表示对某事物非常熟悉以至于能够记住或背诵出来。