Rough meaning is Between (...)
Interval
describes an interval of time, space or something else, just like 間に, but it can only be used when the provided information of time interval is absolutely necessary
子供が寝ているうちにご飯を作った。 While my kid was sleeping, I was making a meal.
(A mother can only make food when she's not looking after the kid, so the time interval is a necessary piece of information)
group of things
３つのうちでは青いのが最も大きい。 Between these three, the blue one is the largest.