An ‘IF / THEN’ order is a way to automatically create either an unattached stop-loss or a take-profit order (the ‘THEN’ part), if an initial order is filled (the ‘IF’ part).
You do this by adding the distance away from your initial order (in pips) that you want either a stop-loss or take-profit order to be placed. If your initial order is filled, then either the stop-loss or take-profit will be created that distance away from the fill price.
Unattached orders are not linked to a particular position and aren’t affected by any changes to your open positions – this means that once it has been created, you will have to manage it separately. If you cancel an order before it is filled, the ‘IF / THEN’ order is also cancelled.