A trailing stop order is a special kind of sell stop order.

Sell stop orders let you say, "If the bid price falls to my trigger price, allow my sell order to execute on an exchange." The key difference between a traditional stop order and a trailing stop order is that the trigger price in a stop order stays the same as when you entered it, but the trigger price for a trailing stop order can move up when the bid price moves up.

In both cases, if the bid price falls to or below the trigger price, your order may execute.