Define "hydrate" for those unfamiliar with the concept

On https://reactjs.org/docs/react-dom.html#hydrate it seems like an understanding of "hydration" is already assumed when describing the function. The text simply defines the term in a circular manner: "hydrate(): Same as render(), but is used to hydrate a container...".
I am not sure I completely understand what "hydration" means in the react/SSR world and hence I cannot submit a PR myself, but it would be nice if a phrase or two would introduce the reader to "hydration", what does it do exactly and why does it exist as separate from render().

Beginne mit der im Issue verlinkten React DOM hydrate-Dokumentation und vergleiche ihre Formulierung mit der render()-Dokumentation. Recherchiere den React-Kontext für serverseitiges Rendering ausreichend, um zu erklären, was Hydration bewirkt und warum hydrate() von render() getrennt ist; die Dokumentation ist fertig, wenn die Definition ohne Vorkenntnisse verständlich und nicht mehr zirkulär ist.