Acid value of oil:

The acid number is defined as the number of milligram of $\text{KOH}$ required to neutralize one gam of oil or fat

Fat may become rancid after long storage

Fat or oil is hydrolysed by different microorganisms with the formation of fatty acid.

Acid value of oil gives information about age of oil sample, also it signifies due to attack of atmospheric oxygen, hot moist air or microorganisms how much generation of free fatty acid has taken place leading to rancidity.