Under Settings > People section, you will see a set of Default fields appear at the top in the blue shaded box. A Blue star denotes a Mandatory field.
You can add custom fields of your choice by clicking on the ADD CUSTOM FIELD button. All custom fields will appear below the default fields.
When adding a custom field, you can choose from 7 different field types. Click on the star icon to mark any field mandatory:
Some common examples of custom fields used are certifications, experience, willingness to relocate, hire date, division...
To learn more about the Settings section click here.