CF = 'conditional formatting' which is part of the built in form design interface.
Setting the colour is done in the conditional formatting dialogue, not in code. The expression merely compares the value of the primary key column to the value of the unbound hidden text box to which the value of the primary key of the currently selected row has been assigned in the form's Current event procedure. Consequently the expression will evaluate to True only for the currently selected row, implementing the conditional formatting to change the colour of the control for that row only.