module Devise::Models

def self.config(mod, *accessors) #:nodoc:

:nodoc:

inside the given class.
To add the class methods you need to have a module ClassMethods defined

3) And an instance method stretches.

which have higher priority than Devise.stretches;
2) Some class methods for your model Model.stretches and Model.stretches=

1) An accessor called Devise.stretches, which value is used by default;

The line above creates:

Devise::Models.config(Devise::Authenticatable, :stretches, 10)

Creates configuration values for Devise and for the given module. 
def self.config(mod, *accessors) #:nodoc:
  class << mod; attr_accessor :available_configs; end
  mod.available_configs = accessors
  accessors.each do |accessor|
    mod.class_eval <<-METHOD, __FILE__, __LINE__ + 1
      def #{accessor}
        if defined?(@#{accessor})
          @#{accessor}
        elsif superclass.respond_to?(:#{accessor})
          superclass.#{accessor}
        else
          Devise.#{accessor}
        end
      end
      def #{accessor}=(value)
        @#{accessor} = value
      end
    METHOD
  end
end