That
Those
adjective, こそあど word
Remember how それ means "that one"? その has a very similar meaning: that. It's used before a noun that's either close to the person you're talking to, or fairly close to both of you. For example, その本 is "that book", and the book in question might be in the hand of the person you're talking to, or it might be on a shelf not far from you both.
その日
that day
そのとき
at that time, at that moment
そのため
for that reason
その人
that person
その人、有名人だよ。
That person is famous.
その日は、いそがしいんだ。
I'm busy that day.
そのとき、ドーンと大きな音がした。
At that moment, there was a loud boom.