This is the 16282nd most frequent Turkish word.
yapmadığı
"Yapmadığı" means "that he/she/it did not do" or "what he/she/it has not done."
Here, 'yapmadığı' indicates the action (not doing) in relation to Ali's homework.
Ali, bugün ödevini yapmadığı için üzgün.
Ali is upset because he didn't do his homework today.
In this sentence, 'yapmadığı' describes the things not done by the person referred to as 'he'.
Onun yapmadığı şeyler hâlâ aklımda.
The things he didn't do are still on my mind.
'Yapmadığı' here refers to the tasks that were not completed by him.