Properties are configuration values managed as key/value pairs. In each pair, the key and value are String values. The key identifies and is used to get the value, just as the variable name is used to get the value of a variable.
Official Oracle manual
Preparing the .properties file for localization:
It is a localization management service for your projects.
We are trying to make the most convenient, fast and cheap system for localization management. If you have any difficulties using the service, please contact support.