gem install bundler installs the bundler gem through RubyGems. You only need to install it once - not every time you create a new Jekyll project. Here are some additional details:

bundler is a gem that manages other Ruby gems. It makes sure your gems and gem versions are compatible, and that you have all necessary dependencies each gem requires.

The Gemfile and Gemfile.lock files inform Bundler about the gem requirements in your site. If your site doesn’t have these Gemfiles, you can omit bundle exec and just run jekyll serve.

When you run bundle exec jekyll serve, Bundler uses the gems and versions as specified in Gemfile.lock to ensure your Jekyll site builds with no compatibility or dependency conflicts.

For more information about how to use Bundler in your Jekyll project, this tutorial should provide answers to the most common questions and explain how to get up and running quickly.