Confusion of the G from the function of [B,G]=sgolay(...)

Hi, I am confused about the meaning of xp((f+1)/2) = (factorial(p)) * g(:,p+1)' * x, which comes from the Matlab Help for sgolay. Matlab tells me that it is the differentiation. However, book tells me that it is the derivative. So what does it represent?

Thanks,

Yang