Short calls, also known as calls sold to open, are betting that the price of the underlying stock stays below the price you sold it for by the expiration of the contract. Short calls have a theoretically unlimited downside, as in, the share price of the underlying can potentially rise far above the price of the sold call. Here’s an example:

Suppose stock XYZ is trading at $23. You forecast a decrease in volatility and a neutral to slightly-decreasing XYZ price.

You sell a $24 Call for $0.50 to express this view. Your breakeven XYZ price at expiry is $24 (strike price) + $0.50 (premium received) = $24.50, but since the time component of your trade plan may not extend all the way to expiration you should be prepared to buy to close at a variety of XYZ prices as the market value of your $24 Call changes.

Because the price of XYZ can increase infinitely in theory, selling a Call without owning shares as collateral or buying a further OTM Call to create a fixed-risk spread carries a high "risk of ruin". This means that an adverse price move, like a surprise earnings beat or buyout that increases price greatly, can cause catastrophic losses.