@Richard-F The important concept about Z-offset is that it the same for every location on the bed of your printer. It's purpose is to tell the firmware the distance between the nozzle and the bed when the Z-probe "triggers". This way, when your slicer tells the printer to go to something like Z=.2mm for the first layer, it will know where that is.
The picture you included is a visual representation of how flat and level your print surface is. The probe measures the distance to the bed at multiple locations (16 places, in your picture). Once the mesh has been created, the firmware will use the data to make small adjustments in Z to keep the tool at the right height above the bed.