The RSS output style is only available for Feed display types. It will display the view as an RSS feed, which is a specialized XML output. This output is not user visible, but can be parsed by feed readers for aggregation. You may supply a description for the RSS feed; most feed readers will display this description along with the contents of the feed. You may also select to use the site's mission statement for the description. Please note that when using RSS views only comes with the one RSS style. There's no style override for RSS. Modules can add more.