This is the 15327th most frequent Turkish word.
söyleyeceği
"That which he/she/it will say."
In this sentence, 'söyleyeceği' refers to possessions of future speech and connects to 'şeyler' (things), indicating 'things to be said'.
Ahmet'in söyleyeceği şeyler çok önemliydi.
The things Ahmet was going to say were very important.
Here, 'söyleyeceği' defines a future performance action (singing) and connects to 'şarkıyı' (the song), suggesting 'song to be sung'.
Partide söyleyeceği şarkıyı çok önceden seçmişti.
He had chosen the song he was going to sing at the party long in advance.
In this usage, 'söyleyeceği' signifies future utterances and associates with 'sözler' (words), indicating 'words to be said'.