What does mapping in yml file means?
YAML is a very simple, text/human-readable annotation format that can be used to store data.
YAML keeps data stored as a map containing keys and values associated to those keys. This map is in no particular order, so you can reorder it at will. Each pair is in the format KEY: VALUE
When the value is a text string enclose the value with quotes. Some examples below:

KEY1: 'HELLO'
HOST_NUMBER: 1