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().

從 issue 中連結的 React DOM hydrate 文件開始，並將其措辭與 render() 文件進行比較。充分研究 React 的伺服器端渲染脈絡，以解釋 hydration 的作用，以及 hydrate() 為什麼與 render() 分開；當定義不需具備先備知識即可理解，且不再是循環定義時，文件工作就完成了。