A Symlink is basically a shortcut from one path to another. So lets say your iTunes music folder is too big for your boot drive. You can get a huge external drive and move the iTunes library to it and create a Symlink in it's original location to redirect to the second drive.
Lets assume your iTunes library is located at /Users/YourUserName/Music/iTunes and your external or secondary drive is at /Volumes/ExternalDrive/iTunes.
1. Move the files from /Users/YourUserName/Music/iTunes to /Volumes/ExternalDrive/iTunes.
2. Once you verified the files were copied to the external or secondary drive you will delete it from /Users/YourUserName/Music/iTunes.
3. ln -s /Volumes/ExternalDrive/iTunes /Users/YourUserName/Music/iTunes
4. This will create Symlink that looks like a folder with a small arrow on the bottom left hand side. This be created at /Users/YourUserName/Music/iTunes and the actual files will be located at /Users/YourUserName/Music/iTunes.
