What is Hydration?

As per wikipedia:

In web development, hydration or rehydration is a technique in which client-side JavaScript converts a static HTML web page, delivered either through static hosting or server-side rendering, into a dynamic web page by attaching event handlers to the HTML elements.

It makes the server rendered website fully interactive.

Major frameworks like Next, Nuxt, Angular use it.

Page load stages. (source: Fireship)