Plugins help expand your site functions.  Always disable things you decide not to use to prevent confusion though.

Most of the popular ones are automatically activated when you register, or upgrade your account. However some advanced options are sitting there, waiting for you to merely click the activate button.

Such as Subscribe, Comments Plus, advanced Forms add-ons and more. To activate a plugin, simply go login to your dashboard and choose

> PLUGINS

Select the one you want, and activate. To help keep your site running fast, deactivate one if you are no longer using it.