Protein, a pale cube with red stripes.
Protein is not a building material. It's a fuel type material necessary for regenerating health. Build a Health Generator using 5x Wood and 2x Copper , then drag some Protein from your inventory to the generator. Press E while standing next to it to recharge your health. Beware that when you are done using the Health Generator and choose to deconstruct it to get your materials back, you won't receive the remaining Protein in the generator. Try to only fill the generator with as much Protein as it requires to bring you back to full health. This is most easily done by filling it with only a few Protein bits at a time.
Things that can be fueled by Protein