A stop loss is an order instruction to your broker: "if price reaches X, close my position at market." It removes emotion from the decision to cut a losing trade, caps your worst-case loss, and is a non-negotiable component of any disciplined trading plan.
Stop losses are not guaranteed fills. In a fast-moving market, especially around economic data releases, your stop can trigger but be executed at a worse price — that is slippage on the stop. Some brokers offer "guaranteed stop-loss orders" (GSLOs) for a small fee, which guarantee the fill at the stop price regardless of gaps or volatility.
Common stop placement strategies include percent-based (risk 1% per trade), volatility-based (using ATR), and structure-based (below recent swing low for longs). The right stop distance depends on your strategy timeframe and the instrument's typical noise range.