This is the 13575th most frequent Russian word.
начинка
Filling or stuffing (used for food, such as pastry or dumplings).
Here 'начинка' is used to denote the edible filling inside the pie.
Эта пирожок с клубничной начинкой очень вкусный.
This pie with strawberry filling is very tasty.
In this context, 'начинка' refers to the internal mechanical or electronic components of a device.
Начинка в этом двигателе была заменена на более новую.
The internal components of this engine were replaced with newer ones.
Here 'начинка' metaphorically indicates the content or substance within a book.