It is just the derivative, but the dx in the denominator of this "fraction" specifically tells you "with respect to x". It becomes more important if you're doing functions of multiple variables. If you have a function f(x, y, z), then f ' (x, y, z) doesn't tell you which derivative to do (wrt x, y or z). However, if you say d/dx[ f(x, y, z)], then you know specifically to do the derivative with respect to x, considering y and z as constants.