A spigot plugin is an add-on for a game server. This game server called Spigot is a modified Minecraft server which provides functionality for these plugins as well as extras.
This provides additional performance optimizations, configuration options and features, whilst still remaining consistent with Vanilla Minecraft game mechanics.
More information on spigot can be found here: https://www.spigotmc.org/wiki/spigot/
This plugin is something I am in the progress of building in my free time. I have a trello set up to see my current progression and desired features (https://trello.com/b/3JFr5Ztf/rpg-server)
The main idea is for a RPG based server compared to the typical survival. This will feature elements like classes, guilds, boss fights, etc. For more information please visit the trello linked above.