Several distros set up your boot device in grub using the UUID. Unfortunately this can cause the output from the df command to display the long uuid (e.g. /dev/disk/by-uuid/0ebcf386-83eb-4639-b1ba-be73a7f60efc) rather than the device name (e.g. /dev/sda1)
To change this edit the file /etc/default/grub on Debian use the command
sudo vi /etc/default/grub
find the line which contains
If it has a # as the first character on the line this character should be deleted then save the file.
Now update the grub using the command
After you reboot the df command will display the device name.