NovaLib introduces a new way for plugins to be developed. Rather than relying on several different APIs and libraries, plugins can now reference just one. This not only makes it easier for developers, but also means server owners need to maintain fewer plugins and thus less time is spent getting updates for every API plugin out there.
Just download the .jar file for your server (either Spigot/Bukkit or Bungee) and drop it into the plugins folder. Restart the server and you are all set!