Read this to understand what wastegate is first and you'll have a much better understanding of why you may need a low or high wastegate map.... Although Cobb really should call it a low WGDC or high WGDC map to be perfectly correct, since the map they label as "low wastegate" actually uses more of the WG.
WGDC = WasteGate Duty Cycle. The more WGDC there is, the more closed
your wastegate door is, and thus the less
your wastegate is being utilized.
If someone says, "you need to wastegate more" that means you need less
WGDC, and vice versa.
After you read that, what I said above will make sense.