To expand a bit on DaleSpam's explanation, the voltage across the resistance of the rod's nichrome wire generates a current in that wire. The motion of the electrons (who get their energy from the voltage potential) generate "collisions" with the atoms in the wire, which transfers energy from the electron's (kinetic) energy to the metal lattice of atoms. The atoms "vibrate" more from this extra energy, and that appears as heat energy. This is kind of a classical description, rather than a full solid state physics description, but hopefully it helps you to picture how the energy from the voltage source is transferred to the driving force on the electrons, and from there to the vibrations in the atomic lattice of the metal of the rod, which is heat energy.