The Aufbau principle let's us build up an atoms electronic configuration by placing electrons into orbitals of every increasing energy. Hund's Rule tells us about how the electrons in an atom should be placed into degenerate orbitals. Technically Hund's Rules say that the ground state configuration should maximize the multiplicity of a configuration. What does that mean? The simple rule is that electrons should be placed into separate orbitals before going into the same orbital. This is because these best represent the configurations that are the lowest energy when we do the quantum mechanical calculation.