What does it mean to change the default font mapping in Flash? It means that you adjust what font is automatically used in your Flash movie's text when no font is selected (or the selected font can't render on the end-user's side). Usually the default is a sans-serif font such as Arial or Tahoma, but you can adjust that.
Simply open Edit->Preferences from the main menu, then select the Text options section. The very first option is a dropdown menu labeled Font mapping default; clicking on it opens a list of all the fonts installed on your computer. Select the default font you'd prefer, then click OK and you're done.