So lets say you Buy ETH at current price of 0.0555
You will be on vacation or will not have access to your computer or internet to monitor the price of ETH for next couple of days
So what you did to ensure that you will not be surprised when ETH price dropped to 0.04, you place a Sell with Conditions
You choose Less than or Greater To and set the price to 0.0541 ETH so if ever ETH price dropped to 0.0541, it will trigger the system to Sell at your desired price without you logging in to your account.
The link I posted above have screen shots so you will better understanding of what I am talking about.