adverb
at that very moment; just then
Used to emphasize that something happened exactly at a particular time, often coinciding with another event. Typically found in written or formal contexts.
折しも、外で大きな雷が鳴った。
Just then, a loud thunderclap sounded outside.
折しもその日は彼の誕生日だった。
As it happened, that day was his birthday.
その時 simply means 'at that time', without the emphatic 'just at that moment' nuance of 折しも.
Compound of 折 (おり, 'time, occasion') + しも (emphatic particle). The particle しも adds emphasis, yielding the meaning 'exactly at that time'.