A stop order (or "stop-loss" order) sits inactive until a stock trades at a specified trigger price, at which point it converts into a market order and executes at the next available price. It's most commonly used to cap a loss on an existing position: a trigger set below the current price on a long position sells automatically if the stock falls that far, without requiring the trade to be watched continuously.
Because a triggered stop order becomes a market order, the fill price isn't guaranteed to be the trigger price — in a fast-moving or gapping market, it can execute meaningfully below (or above, for a stop on a short position) the trigger, for the same slippage reasons a market order can. A stop-limit order combines the two: it triggers into a limit order instead of a market order, capping the fill price but reintroducing the risk that it doesn't fill at all.
Stop orders are a risk-management tool, not a guarantee against loss — they manage how a loss is capped, not whether one can happen.