Adolescence is defined as the period where individuals must stitch together their various roles (student, friend, athlete, daughter/son) into one coherent sense of self.
The "Coherent" Part: It means having a sense of self that stays consistent across different situations. If a teen acts like a completely different person with every new group of friends, they are struggling to form that coherent identity.
The Struggle: This is often called an "identity crisis." Adolescents experiment with different clothes, music, and beliefs to see which ones feel "real." If they fail to integrate these, they experience role confusion, where they feel fragmented or unsure of their place in the world.