Intertextuality is simply put a segment, shot or sequence of a movie used in other movies to create a similar effect on the audience.
We can compare this to a cliche in an advertising industry for example to advertise a juice with fresh forest berries we will use a poster with the bottle in the forest. This is called a cliche because it's often used, many people don't like that kind of repetition but cliche's are called cliche's for a reason.
So what does it have to do with intertextuality?
Intertextuality in theory seems pointless and you might think that it will ruin the movie but using this can create a sense of familiarity. In the result the movie is more enjoyable to watch.
An example of this can be a scene from 'Psycho'. There are many movies using a similar scene e.g. The Stepfather.
There are more scenes in 'The Stepfather' that will seem familiar for example the family running upstairs to hide from the killer or killing the stepfather doesn't actually kill him. We can see those kind of scenes in other horror/thriller movies and it's still entertaining to watch.