I see "stock" here as an adjective, meaning "standard", and the phrase "the stock and PB clocks" as meaning "the stock clock and the PB clock".
So there is a stock clock (the standard clock), and a Precision Boost clock, and this processor can, under the right conditions, adjust its own clock speed to be higher than either of them.
By the way, when you are asked to say where you found it, we like you simply to say something like "From a review of the ... published in ..."
It is helpful also to give a link (as you have done) for anyone who wants to look at more detail, but the bare link alone is not enough. We don't want to have to click on it just to discover where the quote is from.