Flask comes with a built-in development web server, but you shouldn’t be using it in production.
To get cool features like separate processes for each request and static file serving, you need to run an actual web service and a WSGI service in front of your Flask application.
The Flask docs provide several examples on how to set that up. Popular Web Server/WSGI combinations are Apache/mod_wsgi and Nginx/Gunicorn, but there are many other options.