Coke has two main meanings:

1. A solid fuel made by heating coal in a special oven with very little air. This process, called coking, burns off impurities and leaves behind a hard, gray material that burns much hotter than regular coal. Steel mills use coke in blast furnaces to melt iron ore, since regular coal doesn't get hot enough for the job. Think of it like charcoal for a backyard grill, except coke is made from coal instead of wood and burns hot enough to melt metal. Before natural gas became common, many homes used coke for heating too.
2. A shortened, informal name for Coca-Cola, the popular soft drink. When someone says “I'll have a Coke,” they usually mean the famous brown soda. In some parts of America, people use “coke” to refer to any soft drink, the way some people call all facial tissues “tissues” or “Kleenex.” This casual usage of brand names happens when a product becomes so well-known that its name becomes part of everyday language.