Metals are good conductors of electricity because

Metal particles are held together by strong metallic bonds, which is why they have high melting and boiling points. The free electrons in metals can move through the metal, allowing metals to conduct electricity.

Because the electrons in the atom's higher energy levels can separate from the atoms easily, they can be pulled out with very little energy. Because the molecular structure of the metal is such that 'free electrons' are free to move, thus being categorized as electrically conductive.

For example, copper is used for electrical wiring because it is a good conductor of electricity.