The Nutrient Block is a dense block of processed Food from the Aurora. It is part of the remnants of the ship rations eaten aboard the spaceship before its demise. They provide the most food of any single food item in Subnautica.
They can only be found inside Vac-packs, a type of storage that is found on and near the Aurora, and inside the storage unit of Lifepod 5. They can also find them inside of Vac-Packs in the Crash Zone and in Wrecks, and sometimes in other biomes, but mostly lying around in certain rooms in the Aurora.