Hi,
When you are holding a particular stock/F&O/commodity, you fear the losses that can happen when the price starts moving against you. If you place an order to limit such a loss it is called as a Stop Loss order.
SL: If you bought a stock at 100 and you want to limit your losses to 95 which is 5 points. So if the markets come's against you your order should ideally get squared off at 95.
SL-M: Now with the same example what if the market crashes and passes your SL of 95 but does not execute can leave your SL order pending? this can happen if we have sudden fall. So to avoid these situations we use SL-M and keep a trigger price which is 95. Now even if the markets fall by a margin the order will get executed 95 or below.
My Advice: It is always better to use SL-M and avoid risk.
Hope it helped :)