Bundler is a tool used in the Ruby programming language to manage project dependencies. It helps developers specify which libraries their application needs and ensures that the correct versions of these libraries are installed. This makes it easier to maintain and share projects, as everyone can work with the same set of dependencies. When a developer creates a Gemfile, they list the required libraries, and Bundler takes care of installing them. It also creates a Gemfile.lock file that locks the versions of the libraries, ensuring consistency across different environments. This way, developers can avoid conflicts and keep their applications running smoothly.