when a filesystem is "journalled" it means there is an actual journal that the kernel keeps of every atomic disk read/write that it does... the reason is that in a normal filesystem, power loss actually can cut off an OS before or in the middle of writing something to the disk, ending up corrupting everything
a journal tends to help the situation so that even if the computer gets loses power or the system crashes, it can recover the journal later to help recover the filesystem
on a related note, using the command "sync" flushes everything out to disk immediately