Slippage happens when a market-order or a triggered stop-order walks through several levels of the order-book, or when price jumps between the moment you click and the moment the order arrives. It is worst in thin markets, at the open, and during news.
It is a real cost that backtests routinely ignore. A strategy with a small edge can be wiped out by slippage alone.
Example: your stop is at $50.00. A bad headline gaps the stock to $48.50 and your stop fills there. You planned to lose $1 per share and lost $2.50: 150% more than intended.
Related: market-order, liquidity, bid-ask-spread, stop-order, fill