On various forms, some of the data cannot be changed by user input and is displayed solely for information – if this is the case the background of the form field will be coloured a light grey so you know it is not meant to be editable. 

In other cases fields might be meant for another user, in this case the field access will determine who is able to edit a field, if the user completing the form does not fit that criteria then the field will be greyed out. In the example below the employee can view the date field however only a manager can update it:

Further information on Form Field Permissions can be found here.