The source is public. You can run it, read it, and modify it. What you can't do is take Yew Search, host it for other people, and sell it as your own service. That's the entire restriction.
Yew Search behaves exactly like open source for you. Run it, modify it, fork it, share it - for free, forever.
Internal use - running it for your own team or business - is also free. A commercial license only comes into play if you're hosting it for customers or selling it as part of a product.
Open source projects have repeatedly been taken by large cloud providers, rebranded, and sold as a hosted service - with nothing going back to the people who built it. Fair-Code is a deliberate choice to avoid that outcome, following the same model as companies like GitLab, n8n, and Elastic. It keeps the project sustainable without taking anything away from how an individual actually uses the software.