The original copyright term was twenty years. When copyright expires material goes into the public domain where it can be not only enjoyed but used for inspiration in new works. There is literally no copyrighted material today that doesn't use public domain material.
Current rules would allow corporations to extend copyright forever so material would never enter the public domain. The idea is that so long as material has any value the public isn't entitled to use it. Even twenty years is far too long with the fast moving society of today. I propose ten years on general works and five years for software. Additionally, any software which is referenced in a patent application should be ineligible for double protection under copyright.