Stop orders are used when you assume that the price will continue to move in the same direction after a certain moment. For buy orders, these will be levels that are above the current price, while for sell orders they will be below the current price.
For example, let's assume that the USD/CHF currency pair is trading at the level of 0.8030/0.8033.
You think that if the USD/CHF pair rises to the level of 0.8060, then its price is likely to grow further. Thus, you want to open a buy position when the price reaches this resistance level, and not open a position at the current price.
You place a stop order to buy in the amount of 10,000 USD on the USD/CHF pair when the price reaches 0.8060. This means that when the "ask" price of the USD/CHF currency pair reaches the value of 0.8063, the order will be executed. And you will automatically enter the market with a new long position of 10,000 USD on the USD/CHF pair.