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().

Comece pela documentação de React DOM hydrate vinculada na issue e compare sua redação com a documentação de render(). Pesquise o contexto de renderização no servidor do React o suficiente para explicar o que a hidratação faz e por que hydrate() é separado de render(); a documentação estará concluída quando a definição for compreensível sem conhecimento prévio e deixar de ser circular.