TBN is an additive in oil that prevents wear. If you get oil analyzed it’s one of the numbers that show up and will show an oil change needed if low. It stands for something but I don’t know what.
TBN stands for Total Base Number, and it reflects the alkalinity of the oil in question. As the TBN number lowers (more acidic) the oil needs to be changed, typically at or near zero in a perfect world.