What is bundel exec means?

bundle exec is a bundle command to execute a script in the context of the current bundle (the one from your directory's Gemfile)

Your system-wide Gemfile may differ from current Gemfile. You may have newer or older gems with which this script doesn't play nice. This difference in versions can give you weird errors.
Bundle exec helps you avoid these errors. It executes the script using the gems specified in the script's Gemfile rather than the systemwide Gemfile