This is the 3144th most frequent Turkish word.
diyecek
"(Something) to say" or "will say."
Here, 'diyeceğini' is derived from the verb 'demek' (to say) and is used to denote something that is going to be said by someone.
Onun diyeceğini dikkatlice dinledim.
I listened attentively to what he was going to say.
In this sentence, 'diyeceksin' is the future tense of 'demek' for 'you' (singular), asking about what someone will express.
Bu konuda ne diyeceksin?
What will you say about this matter?
Here, 'diyecekleri' indicates the statements or expressions of the society, used as a plural form of 'diyecek'.