What is titration and why to do it.
A good video explaining the chemistry of biodiesel. Basically, the purpose of titrating is to determine the amount of fatty acids in the waste oil. The transesterification process requires a specific minimum amount of catalyst (KOH or NaOH), to go to completion. The amount of acid in the raw WVO (produced by overusing it in the restaurant) determines how much is needed to neutralize it.
Here is another good website, explaining the process.
And here is a calculator.