[Faceman]

The numbers represent the numbers you have to roll on a 20-sided die to make your save vs spell/death/polymorph/...
A save of 6 would mean that you will save at 6,7,8,9,...,20 and fail at 1,2,3,4,5 giving you an 80% chance to save and 20% to fail.
A save of 16 means saving at 16,17,18,19,20 and failing at 1,2,3,...,15 thus giving you only 20% chance of save but 80% chance of failure.
As you can easily see lower numbers are better.
Negative numbers even make sure you get your save and even up for penalties (e.g. from the spell Greater Malison)