It means that something/someone on your computer did a wacky thing.
GPSBabel for mac is shipped as a .dmg that has a single .app - GPSBabelFE, which is the GUI Front End. An OS/X app is "just" a structured directory full of stuff. You can see this in Finder if you do a "show package contents". Down in Contents/ you'll find normal things in Resources/ like icon images and translations. In MacOS/ you'll find the actual executable that starts when you click on the launcher icon (GPSBabelFE) and normally, you'll see a sibling to that, gpsbabel, that's what's actually run when you click "convert".
Somehow, your .app bundle has become broken and gpsbabel isn't where GPSBabelFE expects to find it.
I couldn't find a search function on the list so I'll ask this. I'm running Mac OS X 10.6.8 on a MacPro. I have the latest version (1.44) of GPSBabel. When I click on the GPSBabelFE icon I get the following message:"Error reading format configuration. Check that the backend program "gpsbabel" is properly installed and is in the current PATH. This program cannot continue."What do I need to do to fix this? Thanks.