Plugins are tools that you can add to your site that give you more functionality. There are some pre-installed plugins that you can enable from the plugin menu (under ADMIN).
There are a few plugins that add functionality without affecting other parts of your site. You might as well add them. Others you should only add if you want that specific functionality (Expanding Menus, Movement Script).
You can keep an eye out for more plugins on the Indexhibit page: