Colloquial contraction of 'それじゃ'/'じゃあ', used to say 'well then' or 'bye then' when leaving or transitioning in conversation.
んじゃ、また明日ね。
Well then, see you tomorrow.
んじゃ、会議を始めます。
Right then, we'll start the meeting.
買い物終わった？ んじゃ帰ろうか。
Finished shopping? Well then, shall we go home?