When a server sends HTML to your browser, the page already looks complete before any JavaScript finishes loading.
But looks can be misleading. Buttons don't respond. Forms don't submit. Dropdowns don't open.
Hydration is the process where React attaches JavaScript to that existing HTML, making it interactive instead of rebuilding the page from scratch.
This is also why JavaScript bundle size matters so much. The browser can't hydrate components until it has downloaded, parsed, and executed their JavaScript.
That's why modern frameworks push more work to the server. Not because JavaScript is bad, but because sending less of it usually means users can interact with the page sooner.