A chemically chopped up version of protein coming from bovine placenta (yes, it is an animal-derived ingredient). Proteins are long amino acid chains, so when you chop them up you get peptides (short amino-acid chains) and amino acids, nice molecules that are known as the "building blocks of life".
In general, amino acids, peptides, and proteins are moisturizing, nourishing and regenerating ingredients when you smear them on your face, so is probably Hydrolyzed Placental Protein. According to manufacturer's information, it also works as a film forming, protecting and restructuring agent. Due to its high amino acid content, it improves surface hydration and elasticity and due to its larger peptide and protein molecules, it forms a nice film on the skin that reduces trans-epidermal-water-loss (i.e. hinders water evaporating out of the top layers of the skin). These effects help to maintain the water content of the skin cells and result in a healthy skin & nice complexion.