It's basically a way to restrict search results to particular websites. So you could create a "programming" lens including Stack Overflow and Github, for example, and a search using that lens would only return results from those sites.

Can lenses be shared with other users? And is there some kind of shared repository for them where we could crowdsource for example a lens with the urls of all Python docs of popular packages?

They can be shared in a Team plan, we are still thinking how to do this in a safe way for individual users (to protect against malicious intent).

So how would that apply to academic searches? Do you manually create a lens for the journals and arxiv etc.?

There's actually a built-in Academic lens they provide, although I haven't really used it much, favoring my own "Research" lens. It's not very sophisticated, but since I'm mostly into research related to health, I use this a lot:

arxiv.org, nih.gov, researchgate.net, *.edu

I assume so.

Like Brave Search Goggles: https://search.brave.com/help/goggles

"Goggles enable any individual—or community of people—to alter the ranking of Brave Search by using a set of instructions (rules and filters). Anyone can create, apply, or extend a Goggle. Essentially Goggles act as a custom re-ranking on top of the Brave search index."