It means there isn't a nice little door on the back of the PDA that's designed for you to open so you can pop out the battery and put a new one in (as you would with a mobile phone). There is also no backup battery to preserve your data once the main battery has been removed.
To replace the battery you'd need to first backup the PDA, then unscrew the back case and swap the battery (which has a two pronged quick connector on Clies). Then you'd restore the data to the Clie (which was lost once you pulled out the old battery) and be on your merry way.
-------------------- Lisa Gade Editor in Chief, MobileTechReview