A stop-loss is an order that automatically closes a position when price reaches a predefined level, limiting the loss on a trade to a planned amount. It is the instrument that turns “risk per trade” from an intention into an enforced rule, and it defines 1R, the unit all R-multiple accounting is built on.
Placement is a trade-off. Stops too tight get hit by ordinary noise before the idea can work; stops too wide force tiny position sizes or oversized losses. Most systematic approaches anchor the stop to the level that invalidates the trade idea (beyond the swept low, outside the opening range) rather than to a fixed dollar amount.
Stops are protective, not guaranteed: a stop-market order fills at the next available price, so gaps and fast markets produce slippage beyond the level, while a stop-limit can fail to fill at all. Moving a stop further away mid-trade is the classic way traders convert small planned losses into account-threatening ones.