/media/space /home/<...>/Space ext4 defaults 0 0
OK so I'm assuming that /media/space is a flash drive or some other storage (doesn't really matter).
Before you added that line to /etc/fstab; if you had a directory called /home/<...>/Space
in which you placed music; remember this directory would have been by default mounted on the root
Now, once you've added this line into fstab and rebooted/mounted; you now have /home/<...>/Space
mounted on /media/space
- a different filesystem. This is why you no longer see your music in /home/<...>/Space
If you did something like:
# umount /home/<...>/Space
And then changed into the directory; you'd find your music is still there.