It's used to emphasize that the current situation, topic, or subject is unlike anything that has been discussed before. It signifies a significant shift or change in the conversation or scenario.
Here's an example:
* "We were talking about the new restaurant, but then we got onto a different kettle of fish altogether - politics."
This means the conversation changed entirely from the restaurant to a different topic, politics.
Origin:
The idiom's origin is unclear, but it likely stems from the idea of different kinds of fish requiring different cooking methods and being prepared in different ways. This implies that the subject is entirely different, needing a new approach or perspective.