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

GRUB_DISABLE_LINUX_UUID=true

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

sudo update-grub

After you reboot the df command will display the device name.