Used to change the subject or put a pause on the current topic — equivalent to 'by the way' or 'anyway' in English; common in casual conversation.
それはそうと、明日の会議は何時からだったっけ？
Anyway, what time does tomorrow's meeting start again?
旅行の話はひとまず置いといて、それはそうと仕事の進み具合はどう？
Putting the travel talk aside for now — anyway, how is the work coming along?
それはそうと、週末にみんなで飲みに行かない？
By the way, shall we all go for drinks this weekend?