A stop-loss is an instruction given to a broker to by or sell a stock to limit losses if it moves beyond a certain level.
For example, if you buy a share at 100p but don’t want to bear losses of more than 20p (20%), you might put in a stop-loss so that if the shares fall by more than 20%, they are automatically sold. This doesn’t guarantee they will be sold at 80p, though, just that the order to sell will be given at this point.
Although brokers advise having stop-losses as a safety net, it is not always a good idea to set them at a price that is too close to the price at which you bought them. This is especially important if the share is volatile, because if the reasons you bought the share still hold good, you may find yourself selling when you don’t really need to.