What does 'Course Made Good' in NMEA string mean?

The course made good (CMG), similar to the speed made good (SMG, sometimes also speed over ground), is the direction a vehicle is traveling, independent of the direction the vehicle is pointing (i.e. the heading of the vehicle). It is measured as the angle from true north that you are traveling. The velocity vector of a vehicle would be the combination of the SMG and CMG.

This isn't very important for most land vehicles, since the heading and the CMG (as well as the speed and SMG) are almost always the same. However, sea, air, and space vehicles don't always have a CMG that matches the heading. Space vehicles are the extreme example, where their heading is completely independent of their CMG, and don't have a natural restoring force pointing them back towards their course. Most air and sea vehicles do experience this restoring force due to aero- and hydrodynamics, but can still experience offsets between their heading and CMG (as well as speed and SMG) due to winds an/or water currents.

So why do GPS receivers give you CMG and SMG instead heading and speed through air/water? Well it's simply because GPS receivers don't know this information. CMG and SMG is calculated fix to fix, and because of the way GPS works, they can be computed to a much higher accuracy than a positional fix while still maintaining the same precision. In order to get heading and speed through air/water, you need some kind of compass (usually a gyro compass in larger vehicles, although magnetic compasses are still used) and something like a pitot tube used on aircraft.

If you were using a magnetic compass, this is where the magnetic variation (declination) would come in handy. It's used to correct the value from the compass to represent the angle from true north instead of magnetic north. As for the missing checksum, that just means you can't check to see if the message made it to you without errors.

This site originally written by Dale DePriest and based on NMEA version 3.01 says that field is "Track angle in degrees True." I would interpret it as an azimuth relative to True (Geodetic) North. The missing checksum means that the data can't be verified. There's could be an error in the sentence, but you can't verify the data that's there. In reality, I don't know how often that might occur.