Usually, an application that is using a gem needs to require the gem:
However, if an application is using bundler, which defines the application's gem in a file called "Gemfile":
then the application may invoke bundler in a way that automatically requires all of the gems specified in the Gemfile:
Rails projects use
Bundler.require, so a Rails application will not need to explicitly require a gem in order to use it: Just add the gem to the Gemfile and go.
For more about
Bundler.require, see the bundler documentation
For more about how Rails uses Bundler, see How Does Rails Handle Gems? by Justin Weiss.