A conjunction meaning "if that's the case" or "then"; it follows from the previous statement and indicates what should happen next.
電車が遅れるそうだ。そうしたら早めに出発しよう。
I heard the train is delayed. In that case, let's leave earlier.
彼が来ないと言う。そうしたら私たちだけで始めよう。
He says he won't come. Then let's start with just us.
値段が高すぎる。そうしたら買わない方がいい。
The price is too high. If so, you'd better not buy it.