A private or secret talk shared between people who trust each other; something you don't want others to know (often shortened to 'naisho').
大丈夫、これは内緒話だから他の人には言わないでね。
It's a secret, so please don't tell anyone else.
先生に怒られたことを内緒話にして友達と笑い合った。
We kept it as a secret and laughed about getting scolded by the teacher among friends.
あの二人は内緒話をしているみたいで、こちらに気づかない。
Those two seem to be having a private conversation and haven't noticed us.