This is the 5949th most frequent Turkish word.
tümü
All; entirety.
Here, 'tümünü' is used to refer to the complete amount of an unspecified object, with the suffix '-ünü' indicating possession and object.
Tümünü unuttum.
I forgot all of it.
In this example, 'tümü' is used to emphasize the complete collection of specified items ('kitaplar' - books).
Kitapların tümü masada duruyor.
All of the books are on the table.
The word 'tümü' here highlights the entirety of a determined group ('sınıf' - the class).