Protein are one of the building blocks of life. They are complex molecules built up from amino acid residues. Both animals and plants use proteins, and these can be ingested by humans and used for their own needs. As an energy source, or food, proteins provide the same amount of energy as carbohydrates.
Bacteria also use proteins and can serve as a food source.
Protein can be produced in bioreactors using various type of organisms such as bacteria, yeasts or algae. However as feed-stock is so prevalent on Earth, the technologies used might not be directly transferable to Mars, where there is no ecosystem to produce cheap feed-stock.
Protein can be synthesized directly but this does not appear to be applicable to food production.