Slippage appears when an order is filled away from the quoted price, because the market moved in the interval or because the order was larger than the size available at the best level.
It grows with order size, with volatility and with illiquidity, and all three arrive together in the moments that matter most. A stop-loss triggered in a gap down is filled wherever the next buyer is, which can be far below the stop price.
It is the most common reason a strategy that tested well loses money live. A backtest that assumes fills at the mid price has silently removed a cost that, on an active short-horizon system, can exceed the edge entirely.