Since Ruby 2.6

Bundler was de-facto Ruby’s dependency manager for several years, but became part of the standard library only at version 2.6. Before that, it should’ve been installed separately.

Bundler provides a consistent environment for Ruby projects by tracking and installing the exact gems and versions that are needed.

Since Ruby 2.6, Bundler is a part of Ruby’s standard library.

Bunder is used by creating gemfiles listing all the project dependencies and (optionally) their versions and then using

require 'bundler/setup'

or Bundler.setup to setup environment where only specified gems and their specified versions could be used.

See Bundler website for extensive documentation on gemfiles creation and Bundler usage.

As a standard library inside project, Bundler could be used for introspection of loaded and required modules.

Bundler Reference