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().

从 issue 中链接的 React DOM hydrate 文档开始，并将其措辞与 render() 文档进行比较。充分研究 React 的服务器端渲染上下文，以解释 hydration 的作用以及 hydrate() 为什么与 render() 分开；当定义无需先具备相关知识即可理解，并且不再是循环定义时，文档工作就完成了。