A sentence-ending particle (colloquial, often feminine). With falling intonation it expresses assertion or confirmation (e.g., “That's how it is.”).
今日は休みなの。だからゆっくりするつもり。
I'm off today. So I plan to relax.
そうなの。初めて聞いたよ。
Oh, is that so? I heard that for the first time.
彼女は本当に優しいの。いつも気にかけてくれる。
She's really kind. She always cares for me.
The same sentence-ending particle used with rising intonation to ask a question or seek confirmation (e.g., “Is that so?”).
本当なの？信じられない！
Really? I can't believe it!
そうなの？詳しく教えて。
Oh, is that so? Tell me more.
明日来るのなの？予定が変わったの？
Are you coming tomorrow? Did your plans change?