"Update for GPS programs like Garmin or Mapsource that generates new and better maps."
As you should clearly be able to tell just from its name, Garmin Mapsource isn't actually a program, and doesn't activate when you download and install it. Instead it is an update for GPS devices like Garmin or Mapsource; so you need to first install a version of these programs to be able to apply this update.
What Garmin Mapsource generates and updates is the large number of maps to orient the GPS. It also corrects possible errors in the original maps, adds features and notes, classifies them into new categories, filters the information more effectively, supports external links, etc.
With the update for Garmin or Mapsource you can also make advanced searches, zoom in more precisely on more specific geographic locations, etc. To correctly update the data you simply have to copy the runnable file to the same directory you have the original version in. Once copied, it will run and follow the instructions.