This is the 3376th most frequent Turkish word.
katılacağı
"Katılacağı" means "that he/she/it will participate in" or "that he/she/it will attend."
The word 'katılacağı' here indicates that Ali will participate in the event, referring to a future action and modifying a noun to elaborate the reason for his excitement.
Ali etkinliğe katılacağı için çok heyecanlı.
Ali is very excited because he will attend the event.
'Katılacağı' modifies 'konu', describing the specific topic she is involved with in an upcoming seminar.
Seminerde katılacağı konu üzerine hazırlık yapıyor.
She is preparing on the topic she will participate in during the seminar.
In this case, 'katılacağı' refers to the competition Ayşe is going to participate in, providing information about her commitment to the event.