tl:dr - Testing is awesome. Do it a lot.
Why do we test in Ruby? Ruby is a test-driven development language. Testing inherently shows you where your application should go next. It allows you to specify the behavior that you think should happen before you write code to enact that behavior. One of the most popular methods of testing in Ruby and Rails is RSpec.
RSpec is a behavior-driven development tool for Ruby programmers. Behavior-driven development is an approach to software development that combines test-driven development, domain driven design, and acceptance test-driven planning. RSpec includes a command line program, text descriptions of examples and groups, customized reporting, expectation language, and built-in mocking/stubbing framework.
As a beginner, I’m already thinking:

The error message will point you directly to the file and line number. This will bring up your first test. You don’t really need to know about testing to be able to get through the Koans, you just need to understand that whenever you see a line like this: ___ you need to delete it and fill it in with something else.