According to the RubyMotion guide for getting started, "RubyMotion is a toolchain that permits the development of iOS applications using the Ruby programming language." In less formal terms it lets you write iOS apps in Ruby using your favorite development environment rather than Apple's unpopular XCode IDE. This post assumes you have gone through the guide above but don't have much other iOS development experience.
My number one recommendation for anyone coming to RubyMotion for the first time would be RubyMotion: iOS Development with Ruby by Clay Allsopp. This book has the quality we have come to expect from the Pragmatic Programmers. The code examples are clear and well-documented, encouraging you to work hands-on with RubyMotion from the first chapter. The book website over at PragProg also includes a discussion forum where the author personally answers questions.
The Pragmatic Programmers also put out a RubyMotion screencast. Screencasts are popular within the Ruby-Rails community and seem to already be fairly widespread within the RubyMotion world.
My favorite RubyMotion screencast to date is Jamon Holmgren's tutorial for making an app that displays several YouTube videos. In all I am pretty sure I wrote just over 100 lines of code for this demo app. I am certain that in Objective-C the app would have required much more code, and have been less fun to write. This tutorial uses the ProMotion tool that Jamon also wrote, which helps organize your app behavior based on "screens." You can find a tutorial for getting started with ProMotion here.
If you would like to know a bit more about the background of RubyMotion and the people who use it, there are two podcast episodes I would recommend. The first is the Ruby Rogues interview with Laurent Sansonetti, the creator of RubyMotion. The second is Episode 29 of the Giant Robots Smashing into Other Giant Robots podcast ("The Most Ironic iOS Developer"). In that episode Ben Orenstein interviews two thoughtbot developers, one of whom uses primarily RubyMotion and the other uses Objective-C almost exclusively but has a bit of experience with RubyMotion. They give some nice perspective on the pros and cons of RubyMotion for iOS development and the show notes provide a number of other resources.
Update 1: I forgot to mention that RubyMotion also offers a generous discount for an educational license if you do not plan to sell your apps on the App Store.
Update 2: Jamon Holmgren tweeted a new version of his tutorial this morning (1/30/13).