A stop-loss is a price at which you've pre-committed to exit a losing trade. The point is to define your risk before you enter — once price hits the stop, you're out, no negotiation.
Critical distinction: a hard stop is entered with the broker (limit-on-stop or stop-market) and executes automatically. A mental stop is a number in your head. Mental stops fail catastrophically under stress; this is one of the most documented failure modes in trader journals.
Stop placement is its own discipline. Too tight and normal noise stops you out; too wide and your R-multiple suffers. Common references: just below the prior swing low (for longs), under the moving average that defines the trend, or a multiple of average true range (ATR).
Trailing stops convert a winner into a runner without giving back everything if the move reverses. They're a different discipline than initial stops and need their own rules — moving the stop manually after every bar invites confirmation bias.