The 80-20 rule states that 80 percent of the results are attributable to 20 percent of the causes. It is a guide that allows people to focus their efforts on the 20 percent that is important knowing the other 80 percent is of minor consequence.
A stop loss order is an order placed with a broker to sell a stock immediately if it drops to a certain price. It's a common way for investors to protect themselves from the possibility of a large loss. It is simple and costs nothing. You simply tell your broker you want it.