Core terms
Stop-limit order
An order that becomes a limit order once a trade occurs at or through the stop price. It controls the worst price but may not fill.
A stop-limit has two prices: the stop, which triggers it, and the limit, the worst price you accept. A sell stop-limit with a $4.50 stop and a $4.40 limit becomes a sell limit at $4.40 once $4.50 trades. If the stock drops straight through $4.40, the order does not fill and you are still in the position.
That is the trade-off with a plain stop: the stop-limit protects the price and gives up the guarantee of getting out.
What traders should verify
Set the gap between stop and limit wide enough for the stock's normal spread and speed, or accept that the order may be skipped.
Do not conflate
In a gap or halt reopening, a stop-limit can leave you with the full position at a much worse price.