Bundler is a dependency management tool for Ruby projects. Its own site describes it as providing “a consistent environment for Ruby projects by tracking and installing the exact gems and versions that are needed.” You declare your dependencies in a file called the Gemfile, listing each gem and an optional version constraint, and Bundler installs them for you.
Bundler’s key contribution was the pairing of two files. The Gemfile records what you want in broad terms; running bundle install resolves all of those requirements, including the transitive dependencies of your dependencies, and writes the exact resolved versions into a Gemfile.lock. The official guides explain that committing the lock file lets later installs skip resolution and reproduce an identical set of gems across development, staging, and production. The bundler.io site frames this directly as “an exit from dependency hell.”
The project came out of the Ruby on Rails world. In a November 2009 post announcing the new tool, Yehuda Katz wrote that Bundler lets you “bundle your gems (both pure-ruby and native) with your application,” with automatic dependency resolution and reproducible installs so that a fresh checkout plus one command is all that is needed to get running.
Bundler matters far beyond Ruby because the Gemfile and Gemfile.lock pattern became a widely copied model. The idea of a human-edited manifest plus a machine-generated lockfile that pins exact versions now appears in npm, Composer, Cargo, Poetry, and many other ecosystems.