eco is the development library that powers all eco plugins. It handles a wide range of backend features, and allows eco plugins to be as powerful as they are. However, it means that you have to install it as a separate plugin on your server.
How do I install it?
It's just like any other plugin on your server. Drop it into the /plugins/ folder and restart the server. However, eco also needs ProtocolLib to function - if you don't already have it on your server, make sure to grab the latest version here