In Selenium .properties files are mainly used to store GUI locators / elements, and also Global fields like database configuration details
'.properties' files are mainly used in Java programs to maintain project configuration data, database config or project settings etc. Each parameter in properties file are stored as a pair of strings, in key and value format, where each key is on one line. You can easily read properties from some file using object of type Properties.
Below is a example program which demonstrate to read the data from .properties file using Java.