You need ruby, probably version 2.5-ish, and a newish version of Bundler.
(If bundle install complains that it doesn't know what bundler is, gem install bundler first.)
bundle install within the repository should create a vendor/ directory with all the stuff jekyll needs to make pages.
Now you can run bundle exec jekyll serve and any changes to things other than _config.yaml will be auto-reloaded.
Publishing happens on push to master.