The Gemfile.lock file is an essential part of Ruby projects that use the Bundler tool. It records the exact versions of all the gems and their dependencies that are installed for a project. This ensures that everyone working on the project has the same environment, preventing issues that can arise from version mismatches. When a developer runs the command to install gems, Bundler checks the Gemfile for specified gems and then generates or updates the Gemfile.lock file. This file helps maintain consistency across different machines and environments, making it easier to manage dependencies in Ruby applications.