Java is a programming language. You need something called the Java Runtime Environment to run programs written in Java. The Plugin is part of that. A small number of websites also use this. The Java Runtime Environment will appear in Programs and Features in the Control Panel and can be uninstalled from there. Unfortunately, there is a bug in the uninstallation that leaves the entry for the plugin in the Internet Explorer Manage Addons window after Java is uninstalled. This is harmless because the file has been deleted. To remove the entry in Manage Addons would require editing the Registry.
This bug has been known since at least 2008, but nothing seems to have been done about it.