It's basically a saved partial filter that you can use when searching. So you can e.g. say you only want to search reddit and have that saved as an easily-accessible button. But there's also predefined ones like "Forums" and "Academic", so that's the general idea.
The cool thing is you can enable a Lens when using the AI assistant with internet access, so its searches are also constrained to that Lens.
Lenses are different from kagi small web. I encourage you to read the docs on both.
You are replying on someone explaining lenses which is something google had something similar in the past when they allowed power users to control search.