expression, adverb
before long; soon; someday; sooner or later
Refers to an unspecified future time, often with a sense of 'eventually' or 'in due course'. Used when the exact timing is not important.
そのうち雨がやむでしょう。
It will stop raining before long.
Let's meet again someday.
expression, adverb
among them; of those; of the aforementioned
Used to refer to a subset of a previously mentioned group. Often appears in formal or written contexts.
応募者は10人いたが、そのうち3人が合格した。
There were 10 applicants, and among them, 3 passed.
彼は多くの本を書いたが、そのうちの数冊はベストセラーになった。
He wrote many books, and of those, several became bestsellers.
Compound of その (that) and うち (inside, within). Originally referred to 'within that (time/group)', which developed into the adverbial senses of 'before long' and 'among them'.