This is the 198th most frequent Japanese word.
中身
The contents; the inside; the substance.
In this sentence, '中身' refers to the physical items that are present inside the box.
箱の中身はお菓子でした。
The contents of the box were sweets.
Here, '中身' is used metaphorically to describe the subject matter or essence of the speech.
彼の話の中身はとても興味深かった。
The content of his speech was very interesting.
This sentence uses '中身' to describe the ideas or topics included within the book.