An archetype is a guide to follow, it is a prototype image to duplicate and reproduce; ideally, a pattern that serves as a basis for shaping or perfecting something. Within the cinema, archetypes can be found in film genres and in the tracing of characters as repeated elements that create standards, formulas or patterns.
An archetype is, for example, that character who works as a hero's mentor: for example, Dumbledore for Harry Potter or Obi Wan Kenobi for Luke Skywalker.
The film or the story needs those archetypal instruments to work, because they allow us to capture the values that we want to express and cohesion the characters.