This is the 15019th most frequent Turkish word.
sendikalarının
"Of the unions" or "of the syndicates" (possessive form).
In this sentence, 'sendikalarının' is used in possessive form to indicate the unions belonging to the workers.
İşçilerin sendikalarının toplantısı bugün saat üçte başlayacak.
The meeting of the workers' unions will start today at three o'clock.
Here, 'sendikalarının' again is in possessive form but 'sendikaların' conveys the possessive connection to the plural pronoun 'their'.
Sendikalarının haklarını korumak için birçok mücadele verdiği biliniyor.
It is known that their unions fought many battles to protect their rights.
In this sentence, 'sendikalarının' refers to the suggestions made by the unions which possessively belong to the employees.