ActiveSupport::Concern is a Rails built-in module which adds some magic to the plain Ruby Module. To take usage of it, a module should extend it:

module M
  extend ActiveSupport::Concern
end

What kind of magic we talk about?

1. It defines class_methods method which defines class methods from a given block:

   # A plain module
   module M
     def self.included(base)
       base.extend ClassMethods
     end
   
     module ClassMethods
       ...
     end
   end
   
   # A module with `ActiveSupport::Concern`
   module M
     extend ActiveSupport::Concern
   
     class_methods do
       ...
     end
   end

2. Autoloads module’s dependency in a base class. If Foo module depends on Bar, withouth ActiveSupport::Concern both modules need to be included in a base class:

   module Foo
     def self.included(base)
       base.class_eval do
         def self.method_injected_by_foo
           ...
         end
       end
     end
   end
   
   module Bar
     def self.included(base)
       base.method_injected_by_foo
     end
   end
   
   class Host
     include Foo # We need to include this dependency for Bar
     include Bar # Bar is the module that Host really needs
   end
   

   If both modules extend ActiveSupport::Concern, it’s enough to include Bar only:

   module Foo
     extend ActiveSupport::Concern
     included do
       def self.method_injected_by_foo
         ...
       end
     end
   end
   
   module Bar
     extend ActiveSupport::Concern
     
     include Foo
   
     included do
       self.method_injected_by_foo
     end
   end
   
   class Host
     include Bar # It works, now Bar takes care of its dependencies
   end