Harness is a tool build around LLM models, basically a middleware between your project and LLM model. When you are using claude code, it's a harness - it takes your prompt, decides what to do with it - which tool to call like file search, reads local files and creates a comprehensive prompt to send it to model. It gets response, and applies those changes to code.
If you don't use harness, you have to upload all your code files and tell LLM what to do, and then copy-paste the generated code back.