driving – What do two fingers up mean on roads in California?

As a Californian, I disagree with the previous answer. I think two fingers is is pretty clearly a friendly “hello” wave (it might be accompanied by the word “peace”). In this case, he was likely either saying:

“Yo! Sorry I’m passing you on the right!”

or, if he’s being passive-aggressive:

“I’m not happy that I have to pass you on the right, but I’ll make a friendly-looking wave anyways so that I don’t seem too mean… but, hopefully you take the hint and move over, ~expletive~!