"Human fingernails" refers to the hard, protective covering at the tips of our fingers that grow continuously. They help us with gripping objects, scratching, and are a part of personal hygiene. Full definition
They have no root, so they are are more like human fingernails than teeth. (second-opinion-doc.com)
A nanometre is also about the length human fingernails grow by each second. (newscientist.com)
Murray explains that cat claws can't scratch and iPad because it's the same material as human fingernails, whereas dogs range in size, so the concern was that a larger dog could really do some damage to an iPad. (eurogamer.net)