irb, or Interactive Ruby Shell, is a REPL that ships with Ruby. It allows you to experiment with the Ruby programming language in realtime. You start irb from the command line, enter some Ruby code, and irb prints out the results for you.
You'd have to install Ruby to use irb, but installing Ruby is beyond the scope of this article.
To start the interactive shell run the command irb
and you will see the following output
2.2.0 :001 >