It basically means how much acid neutralizing ability your oil has in it. When an oil is new it'll start out at a certain number, for example it might be 8.5, and as the oil ages and it's additives get used up, this number will drop. Once it gets too low then the oil is not able to fight off the acids as well.
TBN - Total Base Number, a measure of the amount of basic additives in a lubricant. High TBN is desirable in a crankcase oil to control corrosive engine wear from the acids of combustion. May be measured by ASTM test methods D 664 (Total Base Number) or D 2896 (Alkalinity Value). These methods may give different results for a given oil.