A stop loss is a way to tell your computer to get out of a trade automatically if it's losing too much money. It will "stop" your "loss" for you. You don't want to have to sit in front of your computer all the time with your hand on the button just in case something goes wrong.
When you create a pending order, you can set your entry, stop loss, and take profit. Essentially, you're telling the computer, "I want to get in here and then out here if I win and out here if I lose." That way, you can place the order and your account will do all those things automatically.
An active stop loss will look like a horizontal dotted line on your chart.