Help me explain the formula:

"for $\displaystyle f(t,\widetilde{Z}(t))$ we have:

$\displaystyle df=\frac{df}{dt}[dt]+\frac{df}{dz}[dZ]+\frac{1}{2}\frac{d^2 z}{dz^2}[dt]$

$\displaystyle \frac{df}{dt},\frac{df}{dz},\frac{d^2f}{dz^2}$ are usual derivatives from calculus."

Thanks a lot.