Metal are good conductors of electricity because they have large numbers of electrons within their makeup that have energies that allow them to exist in the conduction band. These are the so-called fee electrons that roam the metallic crystal matrix. When we apply a voltage across a metal, the free electrons are available to support the current flow.
Remember that current flow is a "shifting" of charges. If an electron enters one end of a wire, it doesn't not travel the wire and come out the other end. Rather, all the free electrons in the wire "move over" one position, and an electron emerges from the other end. With lots of free electrons, the metals are really good at this