A plugin is a special modification, that changes or adds to the servers functionality.
This is required to extend the use of normal vanilla Minecraft. Many servers use this to offer multiple game modes for their players.
This allows players to play without having to install any kind of additional software, as a plugin is run soley on the server.
To install a server plugin the associated '*.jar'-file has to be placed inside the 'plugins/'-folder and the server has to be restarted or reloaded.
Additional information regarding the installation or use of the plugin is can be read on the project sites.
To use a plugin, a modified server version such as Spigot is required.
This information is supplied without liability.