According to the Cordova documentation:
"A plugin is a package of injected code that allows the Cordova web view within which the app renders to communicate with the native platform on which it runs. Plugins provide access to device and platform functionality that is ordinarily unavailable to web-based apps. All the main Cordova API features are implemented as plugins, and many others are available that enable features such as bar code scanners, NFC communication, or to tailor calendar interfaces."
In other words, Cordova plugins are the window to your device-specific features. The Cordova/Phonegap team has already built the needed plugins to work with almost all device-specific features. There are community-contributed plugins as well that can provide customization wrappers around device-specific features.
Note
You can search for existing plugins here http://plugins.cordova.io/.
During the course of this chapter, we will be exploring a few plugins.