Inertia SSR - Client side hydration

Hi, I would like to ask what does it mean https://inertiajs.com/server-side-rendering that "Client side hydration" part. I was just trying to figure out what changes i can notice in my app, how can i test whats happening, what i can see with that update of app.js and what i can see without updating it. Because even i didnt update that app.js file it seems my SSR was working, even my reactive parts i can see my DOM is being updated.