in it; within that; among them
A demonstrative phrase meaning inside or within something just mentioned. その中に often points to something located inside it; その中で often means within that group, situation, or context, i.e. among them.
When I opened the box, there were old photos inside it.
候補は五人いる。その中で彼が一番若い。
There are five candidates; among them, he is the youngest.