It is an automated order to close your position (sell or buy) if the price touches some value.

For instance suppose you buy Apple at $100 and the market crashes while you wasn't able to what happened and now Apple is worth $40. You've lost a lot.

With a stop loss order you tell your trader to sell Apple at if the price goes under $80.

You still loose money, but you can decide how much money you can afford to loose in a trade.