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().

Start at the React DOM hydrate documentation linked in the issue and compare its wording with the render() documentation. Research the React server-rendering context enough to explain what hydration does and why hydrate() is separate from render(); the documentation is done when the definition is understandable without prior knowledge and no longer circular.