Hydration
Hydration is the process that makes already displayed HTML interactive, by attaching client side JavaScript to it.
The server first sends a complete HTML page, visible right away. JavaScript then loads in the background and attaches interactive behaviors (clicks, forms) to that already present HTML.
This approach, used by frameworks like Astro, combines the fast display of a static site with the interactivity of an application. Only the components that need it get hydrated, which keeps the page light.