Electrons in copper wire have an easy time breaking free from the pull of their atoms, therefore copper is said to have "low resistance." The easier it is for electrons to break free, the better electricity flows, and the lower resistance a material has.

But materials with high resistance usually have fewer electrons available to break away, and often the atoms are closer together. This means a stronger pull on each electron, which makes them slow down. Because the electrons keep fighting to escape, the atoms shake back and forth and heat up.

In light bulbs, we see the glow from that heat as light.