Intertextuality is when there is a media text referenced within another form of media. An example of this is when a music video references a movie or another song within their music video.
For example in Katy Perry's music video 'Last Friday Night' the video has a reference to social media. There is also a reference to the singer Rebecca Black (who stars in the video) and who originally made a song called 'Friday'. Another reference are the two glee actors that feature in the video which makes the video appeal to a much wider audience.
Another example would be Taylor Swift's music video 'Love Story' which is a narrative based video which references the story of Romeo and Juliet. In the lyrics, she references Romeo and even the costume is of around the same era to the play by William Shakespeare.