A stop-loss is an order that automatically exits a position once price hits a preset level, capping your loss on the trade. It is the mechanical enforcement of the one rule that keeps accounts alive: cut losers before they become catastrophes.
Stop vs. stop-limit
A stop (market) order triggers a market order when your level is hit - you will get out, but the fill can slip in fast markets. A stop-limit triggers a limit order, so you control the price - but risk not filling at all if price gaps through. Each trades a different risk: certainty of exit versus certainty of price, the same tension as market vs. limit orders.
Why the "mental stop" fails
A mental stop - "I'll get out if it hits X" - relies on you executing under pressure, which is exactly when discipline collapses. Price hits your level, hope kicks in, you give it "just a little more room," and a small loss becomes a large one. This is the mechanic behind most account-ending drawdowns. A resting stop order removes the negotiation entirely.
A stop you have to remember to honor is not a stop. It is a suggestion you will ignore at the worst moment.
The system answer
The deepest fix is to take the decision out of human hands. Automated systems place and honor a hard stop the instant the thesis is invalidated - no hope, no "one more candle," no renegotiation. That mechanical discipline is the whole point of rules-based execution, and it is why a defined stop pairs so tightly with position sizing to control risk.