RubyMotion is a tool that allows you to write iOS apps in Ruby . Apple-platform software has historically been limited to using Objective-C, a great but otherwise niche language. RubyMotion allows a completely native and performant alternative: Ruby.
The RubyMotion Tutorial will teach you the essential topics of the iOS SDK using idiomatic Ruby whenever possible. It doesn't cover everything, but it will get you on the path to making elegant apps as quickly as possible.
The RubyMotion Tutorial covers only the very basics of iOS development, so you should also consider reading the RubyMotion book. You'll find expanded content in every chapter, completely new and useful examples, and information more up-to-date with RubyMotion than this tutorial.