Ok to make it easy to understand here's an example:
You buy DDD @ $40/Share and you get 100 shares. At the same time make a Sell Ticket w/ a Trailing stop to sell the 100 shares of stock if the following conditions are met
Last Price / Down / 5% - Sell @ Market Price.
So what happens - ok so lets say
DDD keeps going up up to 55$ then negative PR hits, it starts to go down. as soon as the price goes down to 5% below the last price (or 2.75 below 55) @ 52.25 / then your Trailing Stop or Stop Loss Sell ticket kicks in and sells your 100 Shares of DDD @ 52.75 (5% below Last price) Minimizing losses! Later on you find that the stock dropped to 39$ ouch, saved some $$$ there! Buy it back when you feel ready!
This is a stop loss. Think of it as an insurance policy.