literally, "to add water again." Food that has been de hydrated needs to be re hydrated (allowed to absorb water) before you can cook with it.
The addition of body fluids which have been lost from fever, illness, heat, etc.
To restore lost water to the body tissues and fluids. Prompt rehydration is imperative whenever dehydration occurs, from diarrhea, exposure, lack of drinking water, or medication use. Rehydration can be by oral or IV administration of fluids.