If you're using Mistral Vibe installed on your computer, that is a harness. Vibe is one of many AI harnesses.

In general, a harness is something that lets your AI use tools on your computer and lets it work independently while you walk away. (Very simplified definition, not all harnesses are that automated.)

It's a long time since I tried Vibe, but when I did it was one of the worst. Back then it had broken support for MCP or computer use and other tools people now regard as near essential in AI. Hopefully Vibe is better now. Sadly Mistral are a long way behind.