This is the 11399th most frequent Turkish word.
endişesini
"His/her worry" or "his/her concern."
The word 'endişesini' is used to indicate possession of concern, specifically referring to the concern of 'her'.
Onun endişesini anlayabiliyorum çünkü sınav zamanı yaklaşıyor.
I can understand her concern because the exam period is approaching.
Here, 'endişesini' shows possession of concern, directed towards another person's specific feeling of worry.
Bir insan, başkasının endişesini küçümsememelidir.
A person should not belittle someone else's concern.
In this sentence, 'endişesini' refers to the open expression of the concern possessed by the subject.