If you opened a transaction, but your expectations were not met and the price chart went the other way, the income from your transaction is now reflected in the terminal with a "minus" - it is not yet recorded as a loss. In order not to lose too much, traders set up a special line to limit losses, this is called a stop-loss (see). Once the price chart comes to this line, the transaction is automatically closed, not allowing the trader to lose more than he can afford.
Trading in financial markets involves substantial risks, including complete possible loss of investment capital. This activity is not suitable for all investors. High leverage increases the risk (Risk Disclosure).