Ovastacin is a protein in eggs that helps prevent more than one sperm from fertilizing an egg. It breaks down the outer layer of the egg to let one sperm in, then quickly destroys the layer to stop other sperm from getting in. This protein also stops extra sperm from fusing with the egg after fertilization. In simple terms, ovastacin ensures only one sperm can fertilize an egg, preventing multiple fertilizations and making sure fertilization is successful.