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"Therein" is a formal, old-fashioned word meaning "in that place" or "in that thing" (like inside a document). You mostly see it in legal writing, official documents, or very formal speech — for example, a contract might say "the rules therein," meaning "the rules inside that document." You would almost never hear it in normal daily conversation.
在其中（正式用語，常見於法律文件）
原始詞典資料1 筆記錄
以下是外部詞典記錄的用法，原文照列，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
thereinadv/ˈðɛəɹˈɪn/
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in that; in that place; in him/her/it.
- September 1792, George Washington, letter My observation is that whenever one person is found adequate to the discharge of a duty... it is worse executed by two persons, and scarcely done at all if three or more are employed therein.
- "Ha," said Robin, "comest thou from Locksley Town? Well do I know that fair place for miles about, and well do I know each hedgerow and gentle pebbly stream, and even all the bright little fishes therein, for there I was born and bred. Now, where goest thou with thy meat, my fair friend?"
資料來源：英文維基詞典 English Wiktionary（CC BY-SA 4.0）。 原文照列，未經改寫。