Support for the Ruby 2.1 series ended on March 31 2017. See here for details.
Atom is an XML-based document format that is used to describe 'feeds' of related information. A typical use is in a news feed where the information is periodically updated and which users can subscribe to. The Atom format is described in tools.ietf.org/html/rfc4287
The Atom module provides support in reading and creating feeds.
See the RSS module for examples consuming and creating feeds.