Leitmotifs where first introduced by Richard Wagner in his cycle of four operas, der Ring des Nibelungen, which uses hundreds of leitmotifs. He believed in so called gesamtkunstwerk (total artwork) where every part of the artwork is synchronized together to create something even better. Because of this he wrote both the libretto (script for musicals) and the music for each of his stage works. A leitmotif or leitmotiv literally means "leading motif" and is a short, constantly recurring musical phrase associated with a particular person, place, or idea. These leitmotivs are often referred to as themes (altough they are not exactly the same) and can have many variations.

Jaws

For example in jaws the sharks theme is the two-noted-dundundun theme. We as the watchers know that when we here this theme it means that the shark is near, even without having any other evidence.

In these next two videos you can clearly see that the theme is coupled with the shark. In the first video we, as the watchers, might think there is a shark, but there is no theme so it sounds odd and unusual, and it turns out to be a falls alarm. In the second video however we do hear the shark theme and he indeed is there. Later in the movie the producers took advantage of this by not playing the theme when the shark was there making for a jump scare. And although this might be a extreme example, leitmotivs are used in almost every movie or series, even if you don't immediately recognize them.