Though I've never heard of the term it's likely a nutrient-rich substance that contains what is needed to "life sustain". Maybe it was a powder that when mixed

Though I've never heard of the term it's likely a nutrient-rich substance that contains what is needed to "life sustain". Maybe it was a powder that when mixed with water would turn into something that could be consumed. It definitely doesn't sound delicious but hey, when life is on the line, it should not matter much.