This is the 15586th most frequent Turkish word.
tatilleri
"Holidays" or "vacations" (plural form).
Here, 'tatilleri' is used as a plural noun meaning 'holidays', and it is in the object position of the verb 'seviyor'.
Çocuklar tatilleri çok seviyor çünkü okula gitmeleri gerekmiyor.
Children love their holidays because they don't have to go to school.
In this sentence, 'tatilleri' refers to 'holidays', specifically the idea of taking holidays. It is in the object position of the verb 'yapmak'.
Biz tatilleri kışın yapmak istemiyoruz, yazı tercih ediyoruz.
We don't want to take holidays in winter; we prefer summer.
Here, 'tatilleri' again is a plural noun referring to 'the holidays', used in the object position of the verb 'planlamak'.