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 Supply Crates, a type of storage that’s found in the Crash Zone, Wrecks, and sometimes in other biomes, but mostly lying around in certain rooms in the Aurora. Two are also found inside the storage unit of Lifepod 5.
A useful trick is to consume a Nutrient Block when the Food Meter is near 99 (but not 100), this will overflow the meter to up to 174 allowing for an extra long trip before food is required again.