An archetype is a symbol that appears repeatedly in literature, art and mythology. Archetypes are common across cultures and usually repeat familiar patterns.
An example of such an archetype is that of the gods that intervene on the side of the hero. These are particularly common in Greek myths.
In this archetype, the god in question is usually interested in the affairs of humans. However, he does not care about all humans equally. Instead, he sides with some, and plots against others. Generally, this means that the god is not perfect, and that he has negative emotions that we usually associate with humans, such as jealousy or resentment.
These gods usually intervene in the affairs of humans in a way that is impossible for people. They cause supernatural events that only powerful being like them can bring about. Moreover, these gods usually appear in order to move the action forward through events that are too grand to be able to be caused by humans.