I think this is the shaping of the edge of the apron and it is creating another pleat which is altering the folded edge of the apron from the vertical to a non right angled shape. A trapezoid?

You would get two folds together running down the edge of the apron, and behind them is the deep pleat.

It is something akin to a dart in ordinary dress making, but like all things to do with making a kilt is is bigger, longer and takes up more fabric.

You might be glad of all that extra fabric when sitting down as it means that the aprons drop down between your knees.