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().

Commencez par la documentation de React DOM hydrate liée dans l’issue et comparez sa formulation avec la documentation de render(). Faites suffisamment de recherches sur le contexte du rendu côté serveur de React pour expliquer ce que fait l’hydratation et pourquoi hydrate() est distinct de render() ; la documentation est terminée lorsque la définition est compréhensible sans connaissances préalables et n’est plus circulaire.