High Temperatures, acids from sulphurous fuel & nitrogen air form strong acids when mixed with water from the combustion process making engines hostile environments.
TBN / Total Base Number (aka Base Number / BN) are usually calcium or magnesium based “antacid” tablets lube manufacturers put in the oil to neutralise acids protecting engines. Sounds good. However too much TBN can cause ash in the engine, so it is often a careful balance of choosing an oil with a TBN that’s right for your engine to reduce ash but protect from corrosion. An engine oil analysis can help you determine if your oil TBN is working well.