A stop loss (SL) is a sell order you leave in your broker at a price below your buy price. If the price falls to that level, the order is executed and the trade closes by itself, with a loss you know in advance.
It limits how much you can lose on a trade without having to watch the screen. It is the basic tool for managing risk.
Example: you buy a share at $250 and set the stop loss 0.90 % below, at $247.75. If the price falls to $247.75, it is sold and you lose about $2.25 per share, plus commissions.
A stop loss doesn't guarantee the exact price: if the price moves very fast or opens with a gap, the sale may be executed somewhat lower (slippage).
At Reversal Club, every buy signal comes with its stop loss already calculated, 0.90 % below the entry price. The first thing to do after buying is to set it in your broker.