A conversational conjunction (colloquial form of では) used to change topic, give a conclusion, make a suggestion, or close a conversation (e.g., “well then,” “so”).
じゃあ、明日また会おう。
Well then, let's meet again tomorrow.
もう時間だね。じゃあ、失礼します。
It's time. Well then, excuse me.
じゃあ、それで決まりだ。
So that's decided then.