What is a stop-loss?
A stop-loss is a pre-defined price level at which a trade is automatically closed to limit losses. Standard practice: set stop-loss BEFORE entering the trade, based on chart structure (below support for longs, above resistance for shorts). Never move stops further away once set.
More detail
Stop-losses should be based on chart structure, not arbitrary dollar amounts. 'I can only afford to lose $X' is a poor reason for a stop placement.
ATR-based stops (typically 2× ATR) automatically adjust for volatility — wider on volatile names, tighter on quiet ones.
Stop-loss orders are not guaranteed to execute at the stop price — slippage can occur on fast-moving markets and gaps.