Stop market orders are market orders executed after a stop is triggered. They are similar to stop-limit orders–which fill at a given price. Though, the programmatically executed market order will execute instantly. Ensuring execution, but not an exact price nor an exact amount.
The market order executes immediately. Market orders only ensure the amount that you are willing to pay. The amount you receive depends on the amount of liquidity present in the market.
The stop triggers once the price is met. Once the stop triggers, the market order executes, buying or selling an asset.
Why use a stop market order
In most cases, a limit order can be used to enter or exit the market. But, stop market orders do not require liquidity. Nor are they visible in the public order book.
Limit orders require liquidity because they are guaranteed to execute at a given price. Stop market orders do not, as the stop is simply a trigger.
Hiding orders from the public order book gives less information to the market.