Target Function is a criteria to determine what parameter values set is better. You can create your own Target Functions just like a Function analysis technique (select File->New->Target Function from main menu).
Target Function can maximizing (greather value is better) or minimizing (less value is better). Set Maximize Target Function property to Yes to create maximizing Target Function (No to create minimizing Target Function). By default Maximize Target Function property is set to Yes.
System is shipped with the TotalProfitLossTarget target function. It optimizes for the maximum Ppofit (or minimum loss in the case of unprofitable strategies). It calculates total profit/loss of all strategies applied to all symbols on the chart and therefore can be used to optimize portfolios.
See also Creating Function.