Heroku is a cloud-based platform-as-a-service (PaaS). As a platform, it manages container system for building, running and managing apps. It hide away the burdon of server and infrastructure management and let developper concentrate on writting/deploying apps.
In other word, it’s kind of a “DevOps as a service”; for a developper, that means being able to focus on writting code and not worry about build and deployment.