It's basically an order that you place on a lower rate than you purchase the coin at to minimize your loss. It "stops" your loss after a particular rate that you choose when placing a stop-loss order.

For example: If you buy 1 BTC at $2000 and don't want to bear a loss of more than $200 on your purchase, you can put a stop-loss order at $1800. So, if the price drops down to $1800 or below, your coins will automatically be sold so as to minimize your loss.