In a metal, the outer electrons of the atoms are not held to one atom. They are delocalised, which means they are free to move through the whole giant structure.
Metals are good conductors of electricity. When a potential difference is applied, the delocalised electrons carry electrical charge through the metal. Because the electrons can move, an electric current can flow.
Metals are also good conductors of thermal energy. When one end of a metal is heated, the delocalised electrons gain energy and transfer it through the metal, so thermal energy spreads quickly from the hot end to the cooler parts.
