Adolescent is just a fancy word for teenager — someone who is no longer a child but is not yet an adult. The word can also be used as an adjective. For example, a particular clothing line might be geared toward adolescent girls.
This word refers to all things related to teens — teenagers are adolescent, and the adolescent stage is after puberty and before adulthood. Since adults don't have a high opinion of teenagers, this word is also an insult. Movies full of fart jokes and immature shenanigans are called adolescent. If one adult says to another "You're acting very adolescent" that means "You're acting childish." Adolescent means something close to juvenile, another word that refers to young people and childish people.