Free fatty acid
Vegetable oil consists of triglycerides - that is, chains of three fatty acids joined by a glycerol molecule.
The presence of heat and water in a fryer causes some of these fatty acid chains to split from the glycerol molecule - they become "free" fatty acids (FFAs).
Measuring the level of FFAs in the oil is important for biodiesel production because they neutralise the alkali catalyst used to split the triglycerides. This is why waste vegetable oil must be titrated, and extra alkali catalyst added to compensate.
This neutralisation of the free fatty acids by the catalyst also produces unwanted water in the reaction (acid + base -> salt + water).