1/ an alternative framework for RSCs similar to nextjs, tanstack or react router
2/ a rust runtime for javascript, similar to node, deno or bun (except maybe not as general purpose)
The runtime piece is definitely less general-purpose than Node/Deno/Bun. It's optimized specifically for React Server Components with things like streaming, Suspense boundary handling, and server action execution baked in. You wouldn't use it to run arbitrary JS apps.