RVM is the Ruby Version Manager. It allows you to install up to date versions of Ruby, other implementations of Ruby such as jRuby, manage gemsets for different tasks, and is generally a swiss army knife for getting Ruby running on Linux. Yes, Ubuntu and most distros have Ruby packages already, and they might even be installed in the default installation, but they're almost always out of date. Rather than hunting for up to date packages or relying on someone else to manage the package, using RVM lets you very easily control the whole shebang yourself.
To install RVM, you'll need curl. So first sudo apt-get install curl. Next, and this is much simpler than it used to be, run the following command. Note that you don't need sudo for this command as RVM in its default configuration will only ever install things into your home directory (in the ~/.rvm directory).
$ curl -L https://get.rvm.io/ | bash
And there you are, RVM is installed in ~/.rvm. To finish the installation, either close and re-open all command line windows or run source ~/.rvm/scripts/rvm.