A colloquial expression used when trying to recall something; a casual form of 'What was it again?'.
さっき紹介してくれた人の名前、なんだっけ？
What was the name of the person you introduced earlier?
あの映画のタイトル、なんだっけ…思い出せない。
What was the title of that movie… I can't remember.
彼、今度の会議に来るんだっけ？なんだっけ、確認しておくね。
Is he coming to the meeting next time? Let me check—what was it again.