Even though I didn't learn about this in class I done some research on it and here is what I found:
Intertextuality refers to the linkages across texts or discourse events. The meaning of any text or utterance is shaped by what has come before it, as well as in anticipation of future responses.
The concept of intertextuality was first expressed in the work of Russian philosopher and literary scholar Mikhail Bakhtin (1895-1975). The ideas of the Bakhtin Circle came to prominence via the writings of Julia Kristeva.
To sum it up, it basically means to copy! I know right, simple :)