This pattern turns a statement into a guess or assumption that you're fairly confident about but not certain. It's often used when you're seeking agreement or confirmation from the listener, similar to saying 'I bet it's...' or 'Isn't it...?' in English. For example, if you see a friend with a new phone, you might say '高いじゃない？' (That must have been expensive, right?)

The nuance is softer than a direct guess like でしょう, and it often implies you've already inferred something from the situation. It's conversational and friendly, so avoid using it in formal writing or polite speech. In casual talk, you might hear it with a rising intonation to invite the other person's response.

Be careful: this is not the same as じゃない (with falling intonation) which means 'is not'. The meaning here depends heavily on intonation and context, so listen for the rising tone that signals a question or guess.