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

Indication that the input of a multivariable function can change in many directions. Neither one of these derivatives tells the full story of how our function f ( x , y ) f(x, y) f(x,y)f, left parenthesis, x, comma, y, right parenthesis changes when its input changes slightly, so we call them partial derivatives.

365.25 earth days are in the year of a light year