An expression meaning 'in that situation' or 'amid those circumstances', used to introduce an event or contrast relative to what was just mentioned.
経済は厳しい。そんな中で彼は新しい事業を始めた。
The economy is tough. In that situation, he started a new business.
台風で交通が乱れている。そんな中、試験は予定通り行われた。
Transportation is disrupted by the typhoon. Even so, the exam was held as scheduled.
彼女は病気だった。そんな中、仕事を続けるのは大変だっただろう。
She was ill. Given that, it must have been hard for her to continue working.