IRB means "Interactive Ruby Shell", letting us execute ruby expressions from the standart input.
To start, type irb
into your shell. You can write anything in Ruby, from simple expressions:
$ irb
2.1.4 :001 > 2+2
=> 4
to complex cases like methods:
2.1.4 :001> def method
2.1.4 :002?> puts "Hello World"
2.1.4 :003?> end
=> :method
2.1.4 :004 > method
Hello World
=> nil