Mirror Stage

This is an extract from a website (link below). 

"The idea of the "mirror stage" is an important early component in Lacan’s critical reinterpretation of the work of Freud. Drawing on work in physiology and animal psychology, Lacan proposes that human infants pass through a stage in which an external image of the body (reflected in a mirror, or represented to the infant through the mother or primary caregiver) produces a psychic response that gives rise to the mental representation of an "I". The infant identifies with the image, which serves as a gestalt  of the infant's emerging perceptions of selfhood, but because the image of a unified body does not correspond with the underdeveloped infant's physical vulnerability and weakness, this imago is established as an ideal- I toward which the subject will perpetually strive throughout his or her life.

For Lacan, the mirror stage establishes the ego as fundamentally dependent upon external objects, on an other. As the so-called "individual" matures and enters into social relations through language, this "other" will be elaborated within social and linguistic frameworks that will give each subject's personality (and his or her neuroses and other psychic disturbances) its particular characteristics."

http://www.english.hawaii.edu/criticalink/lacan/  

To simplify, when babies see themselves in a mirror  they identify themselves as I. The baby identifies with the image. 

This can be applied to media; people see a representation of a character or another person, they identify with certain aspects of that person and so strive to be like the represented character.