Launching an app in heroku? What is procfile? 'web:' command?
the Procfile tells Heroku what commands should be run (https://devcenter.heroku.com/articles/procfile).
You are able to define difference process types, such as web (the only one which will autostart by default), workers, etc...
So basically a Procfile containing
web: python app.pyis telling Heroku to started a named process called web, and to run python app.py when it starts.
There is Python specific documentation for Heroku at https://devcenter.heroku.com/articles/getting-started-with-python#declare-process-types-with-procfile