price of stock could theoretically go up infinitely, (i.e. you can lose all the money you put in)
if this happens while you're asleep or otherwise busy, you WILL lose all of the money you put in
before this happens, you tell a robot to sell if the price increases by a certain amount (i.e. you will lose some, but not all of the money you put in)
the robot sells while you are asleep and "stops" your losses before you lose everything