I am not a breeder either...
It means that the front end is in complete harmony with the rear end.
The tail is proportionate to the neck.
I will try to explain.
Since tail docking was banned in many European countries, breeders had to deal with 2 new challenges :
1. The shape
and
2. the length of the undocked tails.
(Also the tail set but this was already an issue.)
The standard asks for the tail :
This describes all 3 concerns.
Length, shape AND position of the tail (tail set).
It also gives us a hint about balance appearance - outline.
The
straight and long undocked tail carried a little higher than the level of the back (ie 45 degrees) gives an unpleasant to the eye appearance resembling a much longer dog.
A straight as possible and
a bit short tail, in the correct position accompanied with the correct croup looks like the ideal according to the standard.
For 100+ years we were breeding dogs with docked tails, unable to know what was the shape or the length of them. We didn't care. Now we have to deal with it. It will take time and commitment to breed the correct shape and length.
Where are we at the moment?
Since many excellent dogs have a curly tail (straight at the beginning and then curved forward) a new type is starting to grow into breeders. This "new" type of yorkies have their tails carried higher than 45 degrees (even 90 degrees) giving them a more compact appearance.
When this type of tail is accompanied with a well laid back shoulder, level topline and compact proportions, it gives a pleasing to the eye appearance, even though its a bit off standard.
Sometimes though, the tail set and the curve of the tail are so extreme that the tail almost touches the back. The curve of the tail is apparent in the first few inches. This effect hides the topline, the croup, upsets the overall outline and often is accompanied with a very long neck, resembling a completely different breed.
Here is a link of the FCI World Dog Show 2014 held in Helsinki.
WDS Yorkshire Terriers - YouTube
You can see the different types of tails that I am referring to.