Forward osmosis
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Forward Osmosis is osmosis however the term forward osmosis is used when talking about using contaminated water, including urine, for rehydrating food to differentiate it from reverse osmosis which is also used to provide potable water for rehydrating dehydrated food in a two step process.
When reverse osmosis or other techniques are used to purify water the purified water can be used to rehydrate dehydrated food. This is essentially a two step process. Pure water is separated from contaminants and then the purified water is added to dehydrated food.
We are used to thinking of contaminated water as having a high solute concentration relative to pure water but if you think of the dehydrated food as solute with very little solvent then contaminated water has a relatively low concentration of solute when compared to dehydrated food. If the dehydrated food is separated from the contaminated water with a semipermeable membrane then pure water will move via osmosis from the contaminated side of the semipermeable membrane to the food side of the membrane.
The advantage of using forward osmosis for rehydrating food is that less water needs to be carried or purified when traveling away from reliable sources of potable water. Forward osmosis will even work with urine allowing purified water to essentially be used twice.