Heat level is an approximation of the Scoville Unit Rating. It's a measure of capsaicin, the chemical in peppers responsible for their heat.
In Spanish, cascabel means rattle, which makes sense when you give these round little chiles a shake. Also known as the chile bola, Cascabel chiles are dark burgundy in color and smooth. They have thick flesh and a lingering medium heat, rating at about a 3-4 on a heat scale of 1-10. The flavor of this chile is rich and acidic with a slight smokiness. When toasted, Cascabels produce a wonderful nuttiness.