Nutrient Bars, which is the offical Alliance name, were referred to as foodstuffs by Mal in Serenity (the pilot). Shaped like a gold ingot and wrapped in foil, each bar can provide a month's worth of nutrition for one person (according to the Serenity RPG; we can assume the ones Mal lifted were supersized). They aren't all you'll need (you'll still need water and additional calories), but with the vitamins, minerals, immune supplements and such, eating them will let you survive on a much smaller diet.