This is the 4th most frequent Croatian word.
su
"Su" in Croatian means "are" (third person plural of "to be").
Here, 'su' denotes existence and associates the subject 'knjige' with its location 'Tamo'.
Tamo su knjige koje tražiš.
There are the books you're looking for.
The word 'su' is the third person plural form of the verb 'to be' (biti), used here to express a state of being as friends.
Oni su jako dobri prijatelji.
They are very good friends.
In this sentence, 'su' is used as the linking verb in the present tense, connecting the subject 'Moji roditelji' to the predicate 'uvijek zauzeti'.