The security system allows you to restrict access on files to certain actions, e.g. you can prohibit users from changing any field in a record. You can fine-tune this access down to individual fields for cases where the whole record is too course a restriction. This is done by going to any form that shows the field you want to restrict access to. In the form, right-click on the entry area of the field you wish to control access; a menu like the one below will appear:

Select the Set field/button access item; a form like the one below will open:

You can control the groups that are allowed to see the field via the View button, and those that are allowed to change the field via the Edit button. When a user is not allowed to see a field it is removed from the form (leaving a space where it was). When a user is not allowed to change a field it becomes read-only (and is shown as blue using the normal colour scheme). Having set the access like this, your settings are applied on all forms and browses that reference that field.