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.
I.e. what power users used to do with Google, before it decided to ignore user intent
That's not a simple hard filter but changes what types of results are more or less preferred.
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.
But really, this is 2024. I shouldn't have to manually enumerate things in Google to get it to spit out serviceable results.
And the above nudges are pretty blunt force, compared to how much more it used to respect exact keywords.