I was reading an old thermodynamics textbook and came across this equation describing change in internal energy. What does the (dU/dT)V and (dU/dV)T mean? I think it’s partial derivatives, but I’ve never seen a derivative sub (something). Please help

I was reading an old thermodynamics textbook and came across this equation describing change in internal energy What does the dUdTV and dUdVT mean I think its p class=