If I buy at the same time, why doesn't the execution price match the price on the screen?
First, what it is: A stop-limit order requires you to enter a trigger price and a limit price. Once the market price reaches the trigger price, the system places a limit order at the limit price.
The logic behind it: The thinner the order book and the larger the order, the greater the slippage on a market order tends to be. Slippage is generally much lower for popular trading pairs with high activity.
For reference, the current quote for $BTC is 74553.17, down 0.69% over 24 hours.
Be wary of claims promising stable, high returns.