A dielectric is the material between two charges. It impeds the flow of charge until a certain voltage threshold at which point charge starts flowing.
When I first learned about capacitors I thought that this was the way capacitors were used. They charge up to a strong enough voltage and then the dielectric breaks down and then they charge up again. I thought this was the basis for timer circuits. We need to be careful to separate dielectric breakdown from things that are supposed to happen in electronic circuits.