The employee fields let you define your own individual input fields. This way you can capture additional information about your employees, that are individually important to you. For example "date of joining the company", "reliability" or "special skills".
In order to create an employee field go to "account" -> "settings" -> "employee fields":
Here you click on "+add field" and enter the desired title:
The employee fields will now be shown in the employees profiles. They are only visible to admins and managers. Here you can add additional information about the employees:
Also: by clicking on "Add more employee fields" you can directly access the settings for the employee fields.