This is the 10993rd most frequent Turkish word.
diyeceklerini
"Diyeceklerini" means "what they will say" or "the things they will say."
In this sentence, 'diyeceklerini' is used to indicate 'what he/she has to say', formed from the verb 'demek' (to say).
Onun diyeceklerini önemsemeliyim.
I should consider what he/she has to say.
Here, 'diyeceklerini' refers to 'the things you will say', focusing on future intent indicated in the context.
Sana diyeceklerini söylemeden gitme!
Don't leave without saying what you have to say to him/her!
In this example, 'diyeceklerini' indicates 'what your friends have to say', referring to their utterances or opinions.