It's an application framework for Svelte which replaces Sapper.
It uses Vite[1] for development and has been designed from the start to be able to produce static sites a la Gatsby or Next (Sapper wasn't great at that). Svelte Kit runs SSR on Node JS servers like Sapper did, but also on cloud functions and I think even Cloudflare Workers which do not run Node.
See Svelte as a JavaScript library like React or Angular, which allows you to develop web applications easily.
Now, SvelteKit is bringing the library to the _framework_ level (big big emphasis on "framework", we all have different definitions for what a framework is), which adds easy routing, server-side rendering, optimized JavaScript bundles per page for the client, etc.
Because to start a project you have to spend days and even months researching libraries, the current best practices, etc. and then glue them all together with some monster build system (webpack, etc.) and pray that one library doesn't update to be incompatible, or that the library maintainer loses interest and maintenance, etc. And then if you look at another project you have to learn everything again because they took an entirely different approach of glueing together libraries.
These frameworks are an opinionated take on project structure meant to save time and effort. Can they build anything and everything with JS? No, and they don't make such promises. Can they build most apps and things people want to build, and with a much faster and simpler setup? Absolutely, yes.
But honestly this isn't the thread to fight the framework vs. library fight. That ship sailed _years_ ago after create-react-app got popular.
It uses Vite[1] for development and has been designed from the start to be able to produce static sites a la Gatsby or Next (Sapper wasn't great at that). Svelte Kit runs SSR on Node JS servers like Sapper did, but also on cloud functions and I think even Cloudflare Workers which do not run Node.
[1] https://vitejs.dev/