Good question. Hydration is where Javascript is rendered statically or on the server and the client needs to take over that HTML.

Traditional frameworks like Next.js, Gatsby, Nuxt.js all "fully hydrate" the client.

This means that every bit of HTML that is sent to the is browser is taken over by JS on the client.

This has it's costs but it is done to give interactivity.

Partial hydration is where you are only adding interactivity to the parts of the site that need it... think of it like the good old days of jquery but with a modern front end framework... Svelte.

Here you go: https://en.wikipedia.org/wiki/Hydration_(web_development)