The "Z Order" is somthing used with GUI developement, to be simplistic it's the order in which widgets are placed from back to front. To illustrate, lets say I'm developing with Photon and had a label called 'PtLabelDontTryThis', and an image called 'PtPictureOfUglyDeadMan', and I wanted my label to be directly ontop of the image for effect. The Z Order of PtLabelDontTryThis would be higher then PtPictureOfUglyDeadMan as PtLabelDontTryThis would be on top.