The Performance Indicator tells you how well the Campaign Item is currently performing by analyzing the up-to-date delivery data (depending on the Billing Category).
For running positions, it calculates the expected performance based on current Delivery Data. The field then tells you how much of the initial goal (the Campaign Item Quantity) will be reached if delivery continues at the same rate as until now. If the campaign item is already ended or is ending today, the Performance Indicator shows how much of the quantity in % is met by the reached lifetime delivery data.
Please keep in mind, that this figure is just a prediction of the Line Item's performance and it does not take fluctuations of delivery data across the items runtime into account.