Viakoo shows you the status of the file system on storage volume. One of the parameters that is displayed is "dirty bit". This field shows "not dirty" or "dirty". What this indicates is that the file system on the volume is healthy as far as the operating system can determine. If the indicator shows "dirty" then the file system on the volume has some corruption. It is a binary condition.
This status is controlled by the operating system of the server that writes to the volume.
Depending on what has been corrupted on the volume MS Windows may warn you the volume is corrupt and continue to allow read/write access to the volume.
Or it may refuse to mount the volume on boot up. Windows will not show a disk volume letter if that happens. Windows will send you ONLY one warning about this condition. After boot up a window will appear that indicates the volume is corrupt and needs a check disk, after a short period the window disappears. If you miss the warning you will have no idea why or volume letter Q: (I picked one at random) is missing, or that it ever existed, unless you go do some deep diagnostic digging in Windows.
Server 2012/Windows 8 and newer versions of Windows attempt to resolve file system corruption automatically when they boot. It is a laudible effort that was not present in previous versions, but it is kind of like trying to fix a dent in your car by wiping a polishing cloth on it. It is not nearly enough effort.
Now on to answer the premise of this article. A dirty file system can be as simple as one missing byte of data. As an over simplified example, if that missing information is an end of file marker (EOF), whick is common when a power failure occurs, the operating system does not know much about that chunk of file data. It may write over that piece of data, because it does not know how big the data chunk is from the start of the chunk and where the chunk ends.
If the operating system writes new data to the volume while it is in the corrupt condition it may overwrite valid data. That is bad, it is lost forever.
You get the idea, a corrupt file system on a volume is bad. Fix it as soon as possible to reduce data loss.
Some other things that can be missing or corrupt and cause data loss or the inablity to access the files on a volume:
File owner attribute, file user access security attributes, file system index attribute, file pointers, hidden status, archive status, read-only status, file compressed flag, encrypted file flag...
For more information, refer to Microsoft's article on best practices for volume configuration.