The things contained inside something (e.g. a bag, box); the interior or filling.
そのバッグの中身を見せてください。
Please show me the contents of that bag.
ケーキは見た目は同じだが、中身が違う。
The cakes look the same, but the fillings are different.
箱の中身は全部割れ物だ。
Everything inside the box is fragile.
Substance or content (abstract): the real meaning, substance, or core of something (e.g. an argument, book, or talk).
その本はタイトルは魅力的だが、中身が薄い。
The book has an attractive title, but its content is thin.
彼の話は中身が伴っていない。
His talk lacks substance.
議論の中身を具体的に示してほしい。
I want you to present the substance of the discussion concretely.
(Specialized) The blade of a sword — a technical/older usage referring to the cutting part or edge.
古い刀の中身はよく研がれている。
The blade of the old sword is well polished.