A cone is a way to measure the temperature in your kiln. Many older kilns will have a kiln sitter, which requires a cone to be placed inside the mechanism and will bend when the cone reaches temperature, it will shut off your kiln. Most newer kilns are run by a computer system and a thermocouple. This essentially takes away the need for a cone. However, many potters choose to still place a cone in their kiln to just double check the accuracy of the kiln computer. You can learn more about cones and appropriate firing temperatures here: https://www.ortonceramic.com/en/Pyrometric%20Cones/