This is the 6582nd most frequent Chinese word.
是說
"是說" means "that is to say" or "meaning."
In this sentence, '是說' indicates what the movie is about, functioning similarly to 'is about.'
這部電影是說一個年輕男子的成長故事。
This movie is about the coming-of-age story of a young man.
'是說' in this context serves to elaborate or state a reason for the first clause.
他昨天沒來上課，是說他身體不舒服。
He didn't attend class yesterday, which is to say he wasn't feeling well.
Here, '是說' is used to preface a recollection of something someone mentioned or intended.