Audacity is the program used to export files from garage band into a more usable file type. When the files are exported from Garage and they are usually going to be exported in a .WAV file type. This file type allows for great music quality but it can’t really be used in video editing programs if you plan to sync the audio with video or to edit the audio to make it sound better
The reason Audacity is so widely used is so that you can change the .WAV file to a more usable file type like . MP3. The way to do this is to open Audacity and open the files you wish to convert, Once you open it all you have to do is go to File/Export/ .MP3. That is all you have to do! Now that you have done that you can use the audio files in any way you want. .MP3 files types can then be used in any video editing program or can just be uploaded to a sound sharing website like SoundCloud or YouTube.