Electrons in copper wire have an easy time breaking free from the pull of their atoms. The easier it is to break free, the better electricity flows. Copper has low resistance.

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.