Refers to a particular time in the past — 'at that time' or 'in those days'; often interchangeable with 'then' or 'at that period'.
彼は大学生だった。その頃、毎日図書館で勉強していた。
He was a university student. At that time, he studied at the library every day.
その頃はまだスマートフォンが普及していなかった。
At that time, smartphones had not yet become widespread.
彼女と出会ったのは大学生のころ。その頃は本当に忙しかった。
I met her back when I was a student. I was really busy at that time.