This is the 917th most frequent German word.
beliebt
Popular.
Here, 'beliebt' is used similarly to describe 'teacher,' implying that she is well-liked or admired at her workplace.
Sie ist eine sehr beliebte Lehrerin an ihrer Schule.
She is a very popular teacher at her school.
In this instance, 'beliebt' is again an adjective, here querying the reason for the popularity of the subject, 'series.'
Warum ist diese Serie so beliebt?
Why is this series so popular?
In this sentence, 'beliebt' is used as an adjective describing the subject 'book,' indicating its popularity or favor among a group, 'teenagers' (Jugendlichen).