What does "app.run(host='0.0.0.0') " mean in Flask
To answer to your second question. You can just hit the IP address of the machine that your flask app is running, e.g. 192.168.1.100 in a browser on different machine on the same network and you are there. Though, you will not be able to access it if you are on a different network. Firewalls or VLans can cause you problems with reaching your application.
If that computer has a public IP, then you can hit that IP from anywhere on the planet and you will be able to reach the app. Usually this might impose some configuration, since most of the public servers are behind some sort of router or firewall.
I am reading the Flask documentation.  I was told that with app.run(host='0.0.0.0'), I could make the server publicly available. 
What does it mean ? How can I visit the server in another computer (just localhost:5000 in my own computer) ?