A Stop Order is commonly used by traders of all levels. It is the price for which an order will fill if the market price reaches the stop price or past.
A trader holds a Bitcoin with book value of $10,000 and is watching the market price fall. The trading strategy is to sell if it reaches $9,000 to minimize the downside of the trade.
To do this, set a Stop Order at $9000, and if the market price reaches $9000 then the order will be filled with the next order on the order books.
Note: the order will fill around $8000 ONLY IF the market price reaches $9000.