Usually this phrase is used to respond to someone who has said “I have something to tell you.” (話があるんだ＝あなたに話したいこと[or 話さなければならないこと] がある。) “話” = "あなたに話したいこと（something to tell you）” 「話ってなんだい？」means "So what is it?" (You can even omit なんだいfor this. ) Hope it makes sense:)