Do you have an example where you've seen it used that is confusing you?
Often (I'm certain there are other uses...at least there always seems to be) の中で comes after a class of things to denote "among" or "within" that class.
Example:
スポーツの中で、どれが一番好きなスポーツですか。
"Out of/among all sports, which is your favorite?"