Which host do I need to fill in the database connection?
It will depend on your hosting setup and where the DB is located. The host value can be:


A reachable hostname where the PHP code is running (e.g. localhost for your local server)
An IP address (e.g. 127.0.0.1, the ip address normally used for localhost), 
A fully qualified domain name (e.g. my-data-base-host.example.com)


Consult your hosting provider on retrieving your DB host connection credentials.