Ullage refers to the level of the wine in the bottle, which can be indicative of how the wine has been stored. It is normally describe by the terms below, except where the wine in question is bottled in Burgundy style bottles when ullage is measured in centimetres:

Into Neck (IN):
Considered a 'perfect' fill level for any wine, though not all producers will even fill a bottle this high.
Base of Neck (BN):
This is an excellent fill level and suggests that a wine has been very well stored.
Very Top Shoulder (VTS):
This suggests a slight evaporation or leakage from the bottle. A fill level of 'VTS' would be usual for a wine around 10 years old.
Top Shoulder (TS):
Common fill level for wines of 15-20 years of age. This would suggest good storage throughout the lifetime of the wine.
High Shoulder (HS):
A common level for wines over 20 years of age. This may suggest some loss or evaporation from the bottle. Colour should also be considered as an indicator of the state of the wine.
Mid Shoulder (MS):
This can be indicative of poor storage of a wine, though may not be a cause for concern if a bottle is over 30-40 years old. In younger wines it may suggest that the wine is not drinkable.
Low Shoulder or Below (LS):
Indicative of poor storage. Normally suggests that a wine is not drinkable.
Considered an excellent to very good level for any bottle
A good level for wines between 15 -30 years of age
5cm & over:
Not unusual for wines over 30 years of age. At this level it is probable that the wine will have suffered some oxidation and may be undrinkable.