Slippage refers to the broker’s inability to fulfil your stop-loss order. For instance, if you want to prevent gap fading, you may set a trailing stop loss of 5%. In other words, the broker should sell the asset once it has decreased by 5% – however, if the market is too volatile (too many buyers/sellers at the same time), the broker will be unable to close your order at 5% – which is known as the slippage risk.