This is the 12745th most frequent Turkish word.
istemeleri
"Their act of wanting/requesting."
In this sentence, 'istemeleri' refers to the desire or requirement expressed by someone ('onun') for another to perform an action ('katılmalarını').
Onun derslere katılmalarını istemeleri çok önemli.
Their wanting them to attend classes is very important.
Here, 'istemeleri' indicates a wish or request made by others (implicit subject) for us ('biz') to perform an action ('yapmamızı').
Projeyi baştan yapmamızı istemeleri biraz zordu.
It was a bit difficult for them to want us to redo the project.
In this context, 'istemeleri' expresses the action of requesting or desiring, associated with 'onlar' (they), affecting us ('biz').