Metals are good conductors because they have many free electrons. How are these free electrons obtained?
Take Sodium for an example. Each sodium because of its low electronegativity wants to lose or donate one electron to the other sodium atoms in the metallic matrix. By losing one electron sodium can obtain the stable electron configuration of neon.
Since each sodium atom wants to lose one electron and none of the atoms wants to gain an electron, these single valance electrons are passed rapidly back and forth between all the atoms in the metallic matrix. This creates a "sea" of free electrons that bond and hold the atoms together. This is called metallic bonding.
The free electrons come from the electrically neutral metal atoms. The unwanted electrons created by the metallic bonding are the "free" electrons that make metals good conductors.