ROAS (Return on Ad Spend) is calculated as: ad revenue / ad cost. A 5x ROAS means that for every euro invested in advertising, you generated 5 euros in revenue. It is the primary metric for evaluating ad campaign efficiency, but should be interpreted in the context of profit margins.
You invested EUR 2,000 in Google Ads this month and generated EUR 13,000 in sales from those campaigns. Your ROAS is 6.5x. With a 30% profit margin, your net profit from ads is EUR 1,900.