C means call. When you buy a call, you have the right to purchase the underlying asset at a certain price, called the strike price. If the underlying asset goes up in price, you can exercise your option and buy the asset at the strike price, even though it's now worth more. If the underlying asset goes down in price, you're not obligated to do anything and can let the option expire worthlessly. Call options are used when you think the price of an asset is going to rise.