Customising fields allows you to take control over how information is displayed to candidates and hiring managers, and indicating if the field is mandatory (meaning it must be completed) or not.
The ability to customise fields is restricted by your user permissions and should only be available to those responsible for maintaining and configuring your system.
To grant / remove permissions from users, navigate to Admin > Users from the Eploymenu. Click on the user's name (opening their user profile), then view the General Permissions tab. The permission that allows Users to Customise Fields is System Admin (Drop Down Lists), so ensure only those authorised to be making changes have access to this permission.
Note - This permission also gives you the ability to amend the options available within drop down lists along with several other administrative functions.