Disable is a conditional action that allows you to define when a field “is” or “is not” available to be completed based on a condition. 

When a field is disabled it will be visible but grayed out in the form. 

This is useful when you want the user to see a field on the form but you also want control of when they can enter data in that field. 

An example is shown here, where a description will be required if Yes is selected. The highlighted box on the form is grayed out when No is selected. 

Important consideration: A field can only be disabled by a trigger. Read here about triggers