No Pre-rendering : On the Initial app load, you can see a blank screen nothing was visible because the initial, the first HTML file sent from the browser was not pre-rendered on the server.

• initial the app was not interactive and no content visible.

Hydration:

• it means when the app becomes interactive.
• browser loaded with JS code.
• when the JavaScript is loaded into the browser, your web page becomes hydrated.

With pre-rendering:

• initial, when the app will have visible content but not interactive
• after, hydration the app will be interactive.