In plain language
The workflow carries a small pointer around and only loads the full record or file at the step that uses it.
Example
A trigger passes a file reference, and the upload step hydrates it by downloading the actual file just before sending it.
Related terms
Dehydrate · Object · Payload · API
Common misconception
In front-end web development, hydration means something else: attaching JavaScript to HTML that was rendered on the server.
Implementation guidance
Hydrate as late as possible, cache results that are reused, handle references that no longer exist, and watch rate limits when hydrating many items.