An order that automatically closes your position when price reaches a loss level you define.
A stop loss limits the maximum loss on a trade. It should be placed according to market structure (support, resistance, volatility), not according to how much you feel like risking.
Once the stop is set, you calculate the lot size so that, if it is hit, you lose only the percentage of your account you decided to risk. That is the right order: stop first, lot second.