Don't worry, it confuses everybody.
I'm not sufficiently expert to be able to give a full explanation. But I can suggest a good hunt of the forums - it has been well explained - and I can also one working example from one of my own missions, which might help.
#relevant bit from a script
; CSM is a group
; gl_CSM is a gamelogic although I can't remember exactly what its for
tank1C sidechat "CSM, stand by for movement orders."
~1
"CSMmarker" SetMarkerType "Start"; "CSMmarker" setMarkerColor "ColorGreen"
? count units CSM == 0:goto "dead"
leader CSM sideChat "CSM standing by."
onMap=false
onMapSingleClick {CSM move _pos; leader CSM sidechat "CSM Moving on"; gl_CSM setPos [_pos select 0, _pos select 1,0]; onMap=true; onMapSingleClick {}}
@onMap
#dead
etc etc