Take a look at this code example from Stack Overflow:

module MyModule
  class << self
    def included(base)
      base.extend ClassMethods
  # A normal instance method
  def multiple
    @x * @y
  # A naming convention for methods that will be extended as class methods
  module ClassMethods
    def factory(x)
      new(x, 2 * x)

With a little experiment, you can see the singleton class of MyModule module is #<Class:MyModule>, which is an instance of class Class.

module MyModule; end
MyModule.singleton_class # => #<Class:MyModule>
MyModule.singleton_class.class # => Class

In the first code example, class << self will give you the singleton class of MyModule module, which is #<Class:MyModule>. Then it defines a included method for the singleton class, which then becomes a class method of MyModule module, which will be called whenever the module is included in aother class or module. This way of adding methods to the singleton class will make sure it doesn’t effect any other class/module that’s also inherited from the class Class.