This is the 4th most frequent Serbian word.
су
The Serbian word "су" means "are" (third person plural of "to be").
Here, 'су' is used as a conjugation of the verb 'to be' (бити) in third-person plural, representing a state of being.
Они су срећни.
They are happy.
In this sentence, 'су' is again the auxiliary verb in third-person plural, indicating the presence or location of the books.
Књиге су на полици.
The books are on the shelf.
Here, 'су' serves as a linking verb in the relative clause to connect 'идеје' with 'добре' and describe their quality.