This is the 26th most frequent Japanese word.
その
"That" (referring to something near the listener).
Here, 'その' is used as a demonstrative adjective, referring to a specific book known to the speaker and listener.
その本はとても面白いです。
That book is very interesting.
'その' in this sentence is used to specify a particular point in time that is understood by the context.
その時、私は駅にいました。
At that time, I was at the station.
In this example, 'その' is used to indicate a restaurant that both the speaker and listener are familiar with.