A conversational phrase used to warn about an undesirable consequence if something is not done; 'otherwise' or 'if you don't (do that)'.
早く出発しよう。そうしないと電車に間に合わないよ。
Let's leave early. Otherwise we won't make the train.
今日中にレポートを出して。そうしないと単位が危ないかもしれない。
Turn in your report today. Otherwise you might lose the credit.
窓を閉めておいて。そうしないと蚊が入るよ。
Please close the window. Otherwise mosquitoes will get in.
練習を続けないと上手くならないよ。そうしないと本番で困るよ。
You won't improve unless you keep practicing. Otherwise you'll be in trouble during the real event.