No eyeglass lens material — not even glass — is scratch-proof. However, a lens that is treated front and back with a clear, hard coating does become more resistant to scratching, whether it's from dropping your glasses on the floor or occasionally cleaning them with a paper towel. Kids' lenses, especially, benefit from a scratch-resistant hard coat.
Since a scratch-resistant coating can't completely protect your lenses from wear and tear, do keep your glasses in a cushioned case, and clean them with a microfiber cloth and a recommended cleaning solution. Not all cleaning solutions are the same - some can even damage your lenses unless approved for your specific type of lens and lens coatings. Also, be wary of those products that promise to repair your scratched lenses. They may fill in the cracks of the scratches, but it is nearly impossible for them to make the scratches disappear so the lenses look new again.