O que é hidratação no React?

What is Hydration?

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.

Because the HTML is pre-rendered on a server, this allows for a fast 
"first contentful paint" (when useful data is first displayed 
to the user), but there is a period of time afterward where the page 
appears to be fully loaded and interactive, but is not until 
the client-side JavaScript is executed and event handlers have 
been attached.
KostasX