The covalent bond that helps to stabilize the tertiary structure of a protein is known as a disulfide bond. Disulfide bonds are formed between the sulfur atoms of two cysteine amino acid residues. These bonds are created when two cysteine residues come into close proximity and their sulfur atoms react to form a covalent bond. Disulfide bonds are important for stabilizing the tertiary structure of proteins because they help to hold different parts of the protein together and prevent them from unfolding.