The meaning of intertextuality is when some scenes or ideas are borrowed and used in another film, many films uses this technique.
For instance the shower scene from Psycho, the idea is used in The Stepfather when David grabs a knife and chases Susan around the house, Susan heads up to the bathroom and slashes David in the throat with a piece of glass. He falls into the bath whilst holding on to the shower curtain, the curtain falls. This is just like Psycho when the woman is in the bathroom having a shower she is stabbed to death and dies in the bath holding on to the shower curtain.
This bathroom scene is similarly used in What lies beneath as she is dead in the bath.
The whole idea of using bits from other films helps the audience to identify and realise what they are watching and can make reference to the familiar scenes/ideas.
What interests me is that the women are always the vulnerable ones in the bathroom.