There are two measurement the GF needs; material height and focus height and the difference between the two can be a bit confusing. The focus height is where you want the narrowest part of the beam hits the material. Most of the time (like 99.5% of the time for most people), you want the material and focus heights to be the same, and luckily the GF takes care of that for you. (Sometimes it’s useful to “defocus” the beam, and you can change the focus height manually. If you search the term you can find some good uses for it.)
The exact material height is needed for the GF to focus the camera on the top of your material so you can line up your file correctly, and that’s what set focus is for. (You can also set the height manually if need be, but obviously set focus is much easier.) Even when you don’t use set focus, the GF does its own auto scan and determines the material and focus height, but your file placement may be way off.
As for kerf, this is the best explanation I’ve found. It is tough to wrap your head around though, and each material can need a different kerf and even the same material can have different kerf depending on the file. I don’t have the need to worry about kerf much, but I know there are some useful kerf tools floating around the forum.
Good luck!