If you're over the age of 10 or so, you probably got chills when you saw the first trailer for Star Wars: The Force Awakens, even if you didn't end up liking the movie. That feeling is due to a concept called "intertextuality", and it's everywhere these days. Note: The above video contains some strong language.
As this video from YouTube channel Nerdwriter explains, intertextuality refers to when one piece of media relies on the viewer's knowledge of another. The trailer for The Force Awakens, for example, offers very little compelling information about its story or characters if you're unfamiliar with Star Wars. However, returning fans of the series will recognise stormtroopers, speeders, droids and, best of all, the Millennium Falcon. Each of these are intertextual cues that rely on another movie to carry the emotional weight of the trailer. You can check out the Rogue One trailer and spot some intertextual references for yourself.
Intertextual references don't necessarily need to involve nostalgia either. The Hunger Games plays on moments that readers remember from the book. The Warcraft movie builds on environments and characters from a video game. The Marvel Cinematic Universe references interconnected movies among its various franchises. Deadpool relies heavily on viewers knowing Ryan Reynolds' other superhero movies like X-Men Origins or Green Lantern. Intertextual references can be built on anything that a viewer already knows from some other property.
Of course, none of this is to say that intertextuality is bad on its own. Watchmen, for example, is filled with comic book panels come to life, and it's a great asset of the film. While some franchises can use intertextual references to build up excitement for a mediocre film, others can use it to enhance or explore new areas of a story that viewers want to see. There's nothing wrong with intertextual references, but it's handy to recognise them when you see them.
Intertextuality: Hollywood's New Currency [Nerdwriter]