An informal phrase telling someone that their interference or advice is unwanted—'none of your business' / 'mind your own business'.
その件は自分でできるから、余計なお世話だよ。
I can handle that myself, so that's none of your business.
余計なお世話ですが、もう少し休んだほうがいいと思います。
This may be none of my business, but I think you should rest a bit more.
彼女は『余計なお世話』と一言だけ言って、部屋を出て行った。
She said simply 'None of your business' and left the room.