When ferrets lay flat, they are wanting to rest just like humans do when we lay flat. If they are on their stomach, they could be stretching their back. If they are on their back they might be watching TV.
If he/she does this often, and for long periods of time, it could be a sign that your ferret is depressed. However, if it is only for a short amount of time, and while he is out playing, he might just be tired.