- It doesn't change anything about the flight plan other than the height it will be executed.
The ground offset feature is designed to get the designated overlap on an area that is at a different elevation than the takeoff point. Imagine you want to get 70% overlap for the roof of a building that is 20 meters tall and you want to take off from the ground. Before you would have had to guess at how much higher you would need to put the overlap setting to get that value up there. Now you can use all of the normal controls for overlap and desired GSD/altitude and put in that ground offset to take care of the rest. Basically it just slides the exact flight plan that is shown up and down in altitude.
You can read more here: https://support.dronesmadeeasy.com/hc/en-us/articles/209895406-Ground-Offset