TBN = Total Base Number, Messured as amount of acid it takes to nuetralize the Base in a motor oil. Motor oils have a basic additive like alka seltzer in them to neutralize acids formed during operation as the base or "TBN" drops that will indicate time to chnage out oil or other problems such as over heating.
A new oil can have a TBN of say 8.0 and if you do a oil test and after 3K miles the TBN is down to say 1.0 it is time to change oil out.
Acids form sludge and varnish and will cause bearing wear and corrosion all not good.
The TBN of oil is the measure of the alkaline reserve, or the ability of the oil to neutralise acids from combustion. Severe depletion of the TBN results in acid corrosion and fouling within the engine.
Maintaining a correct alkaline reserve is critical in preventing unnecessary corrosion of the upper piston, piston rings and top end bearing. Additionally, low TBN is indicative of reduced oil detergency.