SetFieldActive() is used to enable/disable form fields used in the current dialog. The disabled field remains visible, but the user cannot edit the value in it:
CALL DIALOG.SetFieldActive("field_list", boolean)
field_list is represented by a string which contain the field qualifier and, optionally, a prefix (e.g., [table_name.]column). The field list can also be represented as a table prefix followed by an asterisk, which means that all the columns present in the table will be taken into account.
There is some specifics about using variables with ui.Dialog methods.
You can learn about them here.
The Boolean argument is represented by 1 (TRUE) or 0 (FALSE) and indicates whether the specified fields are active or not, respectively.