Which keywords belong to the movie titles with the highest popularity?
Which keywords refers to keyword_name; highest popularity refers to max(popularity)
SELECT T3.keyword_name FROM movie AS T1 INNER JOIN movie_keywords AS T2 ON T1.movie_id = T2.movie_id INNER JOIN keyword AS T3 ON T2.keyword_id = T3.keyword_id ORDER BY T1.popularity DESC LIMIT 1