Svelte is a library that binds JavaScript data to HTML/SVG DOM, so that you don't need to write ad-hoc functions to update DOM. You change values in JavaScript variables marked for reactivity, and Svelte's compiled output implements the DOM mutations. Svelte solves this better than React, both performance-wise and syntax-wise. But not so much better as to make people migrate from React, especially now that LLMs can write either. React has more runtime code, and remains uglier to read for humans: there are paired setter functions, and often-confusing hook semantics.
SvelteKit runs your Svelte code on the server as well (using Node.js), hosting multiple routes (paths), which are first delivered by the server, then updated and navigated by the client. SvelteKit is a superset of Svelte; there's rarely a need to use Svelte without SvelteKit.
Svelte/SvelteKit isn't an all-in-one framework; it considers some things out of scope: data persistence, authentication, CSS framework (beyond the per-component