It means the initial server-rendered page comes with a JSON representation of the store used to render it. Usually in the HTML. The client "rehydrates" this da

It means the initial server-rendered page comes with a JSON representation of the store used to render it. Usually in the HTML. The client "rehydrates" this data into its own store so that its state is consistent with the data rendered. Essentially we're in the same position as if we'd rendered it client-side. It's a lot simpler than it might sound.