Why do metals conduct electricity?

To answer this question we need to think about the bonding that occurs within metals. Metallic elements form strong lattices due to the metallic bonding. This consists of a lattice of positive metal atoms. These metal ions are positive because the negative electrons that normally exist within a metal atom have become delocalised such that they can move around the lattice. This is referred to as a 'sea of electrons'.  Given that these electrons are delocalised and are able to carry charge through the metal lattice, this explains why metals can conduct electricity.