A0 = stores the map the user is currently on
A1 = stores the current X co-ord of the user
A2 = stores the current Y co-ord of the user
FormatStr puts it all into 1 string
%S fetches the data from the end of the command, and they are fetched in the order you see them.. so "%S,%S,%S" %A0 %A1 %A2 would fetch A0 first and put it in place of the first %S, it would fetch A1 second and put it in place of the Second %S etc.
e.g If I am map 0 (Bichon) at 75,100 the string would look like this
"0,75,100,0"
Now we do the MonGenP command which spawns mobs at a specific Map,X,Y,Radius..
So
Map = 0
X = 75
Y = 100
Radius from Co-ord = 0
Weaker Mob = MOb Name
Qty = 1
So the Final command if you had written it without variables would look like this
MonGenP "0,75,100,0" WeakerMob 1
Does that make sense?