Heroku is a "cloud application platform". In short, it provides a CLI tool to add a git remote to your project, and whenever new changes are pushed to that remote, it will analyze the project, try and determine a "buildpack" to run to build the project, and stand it up on one of Heroku's servers.
BloomBus-Client was created using the create-react-app package from Facebook, which is a very popular tool for bootstrapping a basic React application without having to write all of the boilerplate yourself. The community-made buildpack for create-react-app apps on Heroku is used for deployment.