What is hydration?
Hydration is the process of transferring serialized state and dependencies from the server to the static HTML in the client. The HTML can be generated with a static site generator at buildtime, on the fly -meaning because of an event took place on the client-side, that triggered a request for additional HTML- or was pre-rendered and sent from the server -or a caching layer-.
This is the most common method, used by most frameworks that support server-side rendering. When full hydration is implemented, the whole application is bootstrapped, typically during pageload, destroying and recreating the whole Document Object Model -DOM- tree, while state is evaluated and/or data is transferred.